See also:

• HELIOMETER (from Gr. iXtos, sun, and p.Erpov, a measure)

HELIOMETER (from Gr. iXtos, See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun, and p.Erpov, a measure)  , an See also:

• INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument originally designed for measuring the variation of the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun's See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter at different seasons of the See also:

• YEAR

year, but applied now to the See also:

• MODERN

modern See also:

• FORM (Lat. forma)

form of the instrument which is capable of much wider use . The See also:

• PRESENT

present See also:

• ARTICLE (from Lat. articulus, a joint)

article also deals with other forms of See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double-See also:

• IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image See also:

• MICROMETER (from Gr. wcp6c, small, jArpov, a measure)

micrometer . The See also:

• DISCOVERY

discovery of the method of making See also:

• MEASURES

measures by double images is stated to have been first suggested by O . See also:

• ROEMER, FRIEDRICH ADOLPH (1809-1869)
• ROEMER, OLE (Latinized OLAUS) (1644—1710)

Roemer about 1768 . But no such See also:

• SUGGESTION

suggestion occurs in the Basis Astronomiae of See also:

• PETER
• PETER (Lat. Petrus from Gr. irfpos, a rock, Ital. Pietro, Piero, Pier, Fr. Pierre, Span. Pedro, Ger. Peter, Russ. Petr)
• PETER (PEDRO)
• PETER, EPISTLES OF
• PETER, ST

Peter Horrebow (See also:

• COPENHAGEN (Danish Kjobenhavn)

Copenhagen, 1735), which contains the only See also:

• WORKS

works o of Roemer that re- See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main to us . It would appear that to Servington See also:

• SAVARY, ANNE JEAN MARIE RENE, DUKE

Savary is due the first invention of a micrometer for measurement by double image . His See also:

• HELIOMETER (from Gr. iXtos, sun, and p.Erpov, a measure)

heliometer (described in a See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

paper communicated to the Royal Society in 1743, and printed, along with a See also:

• LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

letter from See also:

• JAMES
• JAMES (Gr. 'IlrKw,l3or, the Heb. Ya`akob or Jacob)
• JAMES (JAMES FRANCIS EDWARD STUART) (1688-1766)
• JAMES, 2ND EARL OF DOUGLAS AND MAR(c. 1358–1388)
• JAMES, DAVID (1839-1893)
• JAMES, EPISTLE OF
• JAMES, GEORGE PAYNE RAINSFOP
• JAMES, HENRY (1843— )
• JAMES, JOHN ANGELL (1785-1859)
• JAMES, THOMAS (c. 1573–1629)
• JAMES, WILLIAM (1842–1910)
• JAMES, WILLIAM (d. 1827)

James See also:

• SHORT, FRANCIS JOB (1857– )

Short, in Phil . Trans., 1753, p . 156) was constructed by cutting from a See also:

• COMPLETE

complete See also:

• LENS
• LENS (from Lat. lens, lentil, on account of the similarity of the form of a lens to that of a lentil seed)

lens abed the equal portions aghc and acfe (fig . I) . The segments gbh and efd so formed were then attached to the end of a See also:

• TUBE (Lat. tuba)

tube having an See also:

• INTERNAL

internal diameter represented by the dotted circle (fig . 2) .

The width of each of the portions aghc and acfe cut away from the lens was made slightly greater than the See also:

• FOCAL

focal length of lens X tangent of sun's greatest diameter . Thus at the See also:

• FOCUS (Latin for " hearth " or " fireplace ")

focus two images of the sun were formed nearly in contact as in fig . 3 . The small See also:

• INTERVAL

interval between the adjacent limbs was then measured with a See also:

• WIRE (A.S. wir, a wire; cf. Swed. vire, to twist, M.H.G. wiere, a gold ornament, Lat. viriae, armlets, ultimately from the root wi, to twist, bind)

wire micrometer . ments aghc and acfe are utilized by cementing their edges gh and of together (fig . 4), and covering all except the portion indicated by the unshaded circle . Savary expresses preference for this second See also:

• PLAN
• PLAN (from Lat. planes, flat)

plan, and makes the pertinent remark that in both these See also:

• MODELS

models ` the rays of red See also:

• LIGHT

light in the two See also:

• SOLAR, SOLLER (Lat. solarium, Fr.• galetas, Ital. solaio)

solar images will be next to each other, which will render the sun's disk more easy to be observed than the See also:

• VIOLET

violet ones." This he mentions " because the glasses in these two sorts are somewhat prisinatical, but mostly those of the first See also:

• MODEL (0. Fr. modelle, mod. modele; It. modello, pattern, mould; from Lat. modus, measure, standard)

model, which could there-fore See also:

• BEAR
• BEAR, BLACK
• BEAR, BROWN
• BEAR, GRIZZLY
• BEAR, ISABELLINE
• BEAR, WHITE

bear no See also:

• GREAT

great See also:

• CHARGE
• CHARGE (through the Fr. from the Late Lat. carricare, to load in a carrus or wagon; cf. " cargo ")

charge (magnifying See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power)." A third model proposed by Savary consists of two complete lenses of equal focal length, mounted in cylinders See also:

• SIDE
• SIDE (mod. Eski Adalia)

side by side, and attached to a strong See also:

• BRASS
• BRASS (O. Eng. braes)

brass See also:

• PLATE

plate (fig . 5) . Here, in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order to fulfil the purposes of the previous models, the distance of the centres of the lenses from each other should only slightly exceed the tangent of sun's diameter X focal length of lenses . Savary dwells on the difficulty both of procuring lenses sufficiently equal in focus and of accurately adjusting and centring them . In the Mem . Acad. de See also:

• PARIS
• PARIS (also called ALEXANDROS)
• PARIS, ALEXIS PAULIN (1800-x881)
• PARIS, BRUNO PAULIN GASTON (1839-1903)
• PARIS, FRANCOIS DE (169o-1727)
• PARIS, LOUIS PHILIPPE ALBERT
• PARIS, TREATIES OF (1814-1815)

Paris (1748), See also:

• PIERRE

Pierre See also:

• BOUGUER, PIERRE (1698-1758)

Bouguer describes an instrument which he calls a heliometer .

See also:

• LALANDE, JOSEPH JEROME LEFRANCAIS DE (1732-1807)

Lalande in his Astronomie (vol. ii. p . 639) mentions such a heliometer which had been in his See also:

• POSSESSION
• POSSESSION (Lat. possessio, possidere, to possess)

possession from the year 1753, and of which he gives a See also:

• REPRESENTATION

representation on Plate See also:

• XXVIII

XXVIII., fig . 186, of the same See also:

• VOLUME

volume . Bouguer's heliometer was in fact similar to that of Savary's third model, with the important difference that, instead of both See also:

• OBJECT

object-glasses being fixed, one of them is mcvable by a See also:

• SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

screw provided with a divided See also:

• HEAD (in 0. Eng. heafad; the word is common to Teutonic languages; cf. Dutch hoofd, Ger. Haupt, generally taken to be in origin connected with Lat. caput, Gr. KerbOvi7)
• HEAD, SIR EDMUND WALKER, BART
• HEAD, SIR FRANCIS BOND, BART

head . No See also:

• AUXILIARY (from Lat. auxilium, help)

auxiliary filar micrometer was required, as in Savary's heliometer, to measure the interval between the limbs of two adjacent images of the sun, it being only necessary to turn the screw with the divided head to See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change the distance between the object-glasses till the two images of the sun are in contact as in fig . 6 . The See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences of the readings of the screw, when converted into arc, afford the means of measuring the See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

variations of the sun's apparent diameter . On the 4th of See also:

• APRIL

April 1754 See also:

• JOHN
• JOHN (1167–1216)
• JOHN (1290-c. 1320)
• JOHN (1296-1346)
• JOHN (1371–1419)
• JOHN (1468-1532)
• JOHN (1801-1873)
• JOHN (Heb. llni')
• JOHN (ZAPOLYA) (1487-1540)
• JOHN, 4TH MARQUESS OF TWEEDDALE (c. 1695-1762)
• JOHN, DON (1545-1578)
• JOHN, DON (1629–1679)
• JOHN, GOSPEL OF ST
• JOHN, or HAYS (1513–1571)
• JOHN, THE APOSTLE
• JOHN, THE EPISTLES OF

John See also:

• DOLLOND, JOHN (1706—1761)

Dollond communicated a paper to the Royal Society of See also:

• LONDON

London (Phil . Trans., vol. xlviii. p . 551) in which he shows that a micrometer can be much more easily constructed by dividing a single object-See also:

• GLASS
• GLASS (O.E. glees, cf. Ger. Glas, perhaps derived from an old Teutonic root gla-, a variant of glo-, having the general sense of shining, cf. " glare," " glow ")
• GLASS, STAINED

glass through its See also:

• AXIS (Lat. for " axle ")

axis than by the employment of two object-glasses . He glass so divided still produces a single image of any object to which it may be directed, provided that the See also:

• OPTICAL

optical centres of the segments are in coincidence (i.e. provided the segments retain the same relative positions to each other as before the glasswas cut); (2) that if the segments are separated in any direction two images of the object viewed will be produced; (3) that the most convenient direction of separation for micrometric purposes is to slide these straight edges one along the other as the figure on the margin (fig . 7) represents them: " for thus they may be moved without suffering any false light to come in between them; and by this way of removing them the distance between their centres may be very conveniently measured, viz. by having a See also:

• VERNIER, PIERRE (c. 1580–1637)

vernier's See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

division fixed to the brass See also:

• WORK

work that holds one segment, so as to slide along a See also:

• SCALE
• SCALE (1) A

scale on the plate to which the other See also:

• PART

part of the glass is fitted." Dollond then points out three different types in which a glass so divided and mounted may FIG .

7. be used as a micrometer: " I . It may be fixed at the end of a tube, of a suitable length to its focal distance, as an object-glass,—the other end of the tube having an See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye-glass fitted as usual in astronomical telescopes . " 2 . It may be applied to the end of a tube much shorter than its focal distance, by having another See also:

• CONVEX

convex glass within the tube, to shorten the focal distance of that which is cut in two . " 3 . It may be applied to the open end of a reflecting See also:

• TELESCOPE

telescope, either of the Newtonian or the Cassegrain construction." Dollond adds his See also:

• OPINION (Lat. opinio, from opinari, to think)

opinion that the third type is " much the best and most convenient of the three "; yet it is the first type that has survived the test of See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time and experience, and which is in fact the modern heliometer . It must be remembered, however, that when Dollond expressed preference for this third type he had not then in-vented the achromatic object-glass . Some excellent See also:

• INSTRUMENTS

instruments of the second type were subsequently made by Dollond's eldest son Peter, in which for the " convex glass within the tube " was substituted an achromatic object-glass, and outside that a divided negative achromatic See also:

• COMBINATION (Lat. combinare, to combine)

combination of See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long focus . In the See also:

• FINE

fine example of this instrument at the Cape See also:

• OBSERVATORY

Observatory the movable negative lenses consist of segments of the shape gach and acfe (fig . I) cut from a complete negative achromatic combination of 84 in. See also:

• APERTURE (from Lat. aperire, to open)

aperture and about 41 ft. focal length, composed of a double See also:

• CONCAVE

concave See also:

• FLINT
• FLINT, AUSTIN (1812-1886)
• FLINT, or FLTNTSHTRE (sir Gallestr)
• FLINT, ROBERT (1838- )
• FLINT, TIMOTHY (1780-1840)

flint lens and a double convex See also:

• CROWN

crown . This was applied to an excellent achromatic telescope of 3i-,-, in. aperture and 42 in. focal length . In this instrument a considerable linear relative See also:

• MOVEMENT

movement of the divided lens corresponds with a comparatively small separation of the double image, so that See also:

• SIMPLE

simple verniers See also:

• READING

reading to 'e in. are sufficient for measurement .

With one of these instruments of somewhat smaller dimensions (telescope 22 in. aperture and 32 ft. focus), See also:

• FRANZ, ROBERT (1815-1892)

Franz von Paula Triesnecker made a See also:

• SERIES (a Latin word from serere, to join)

series of measurements at the observatory of See also:

• VIENNA (Ger. Wien; Lat. Vindobona)
• VIENNA, CONGRESS OF (1814-1815)

Vienna which has been reduced by Dr Wilhelm Schur of Strasburg (Nova Acta der Ksl . Leop.-See also:

• CAROL (O. Fr. carole)

Carol . Deutschen Akademie der Natursforscher, 1882, xlv . No . 3) . The See also:

• ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle between the stars and g Ursae maj . (7o8".55) was measured on four nights; the probable See also:

• ERROR (Lat. error, from errare, to wander, to err)

error of a measure on one See also:

• NIGHT

night was 0"•44 . See also:

• JUPITER

Jupiter was measured on eleven nights in the months of See also:

• JUNE

June and See also:

• JULY

July 1794; from these measures Schur derives the values 35".39 and 37".94 for the polar and See also:

• EQUATORIAL

equatorial diameter respectively, at mean distance, corresponding with a See also:

• COMPRESSION

compression 1/14.44 . These agree satisfactorily with the corresponding values 35"•21, 37"'60, 1/15'59 afterwards obtained by F . W . See also:

• BESSEL, FRIEDRICH WILHELM (1784-1846)

Bessel (Konigsberger Beobachtungen, xix . 102) .

From a series of measures of the angle between Jupiter's satellites and the See also:

• PLANET (Gr. ssXavirrns, a wanderer)

planet, made in June and July 1794 and in See also:

• AUGUST (originally Sextilis)

August and See also:

• SEPTEMBER (Lat. septem, seven)

September 1795, Schur finds the See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass of Jupiter =l/Io48.55~I.45, a result which accords well within the limits of its probable error with the received value of the mass derived from modern researches . The probable errors for the measures of one night are o"•577, to"'889, to"•542, 1''.o96, for Satellites I., II., III. and IV. respectively . Considering the accuracy of these measures (an accuracy far surpassing that of any other contemporary observations), it is somewhat surprising that this form of micrometer was never systematically used in any sustained or important astronomical researches, although a number of instruments of the See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind were made by Dollond . Probably the last example of its employment is an observation of the transit of See also:

• MERCURY
• MERCURY (MERCU1uus)
• MERCURY (symbol Hg, atomic weight = 2oo)

Mercury (See also:

• NOVEMBER (Lat. novem, nine)

November 4, 1868) by See also:

• MANN, HORACE (1796-1859)

Mann, at the Royal Observatory, Cape of See also:

• GOOD, JOHN MASON (1764-1827)

Good See also:

• HOPE, ANTHONY
• HOPE, THOMAS (c. 1770-1831)

Hope (Monthly Notices R.A.S. vol. See also:

• XXIX

xxix. p . 197-209) . The most important part, however, which this type of instrument seems to have played in the See also:

• HISTORY

history of See also:

• ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

astronomy arises from the fact that one of them was in the possession of Bessel at See also:

• KONIGSBERG (Polish Krolewiec)

Konigsberg during the time when his new observatory there was being built . In 1812 Bessel measured with it the angle be tween the components of the double See also:

• STAR

star 61 Cygni and observed the great See also:

• COMET (Gr. Koµieiis, long-haired)

comet of 1811 . He also observed the See also:

• ECLIPSE (Gr. €uheo,tis, falling out of place, failing)

eclipse of the sun on May 4, 1818 . In the discussion of these observations (Konigsberger Beobacht, See also:

• ABT, FRANZ (1819-1885)

Abt . 5, p. iv.) he found that the See also:

• INDEX

index error of the scale changed systematically in different position angles by quantities which were See also:

• INDEPENDENT

independent of the direction of gravity relative to the position angle under measurement, but which depended solely on the direction of the measured position angle relative to a fixed See also:

• RADIUS

radius of the object-glass . Bessel attributed this to non-homogeneity in the object-glass, and determined with great care the necessary corrections . But he was so delighted with the See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general performance of the instrument, with the sharpness of the images and the possibilities which a kindred construction offered for the measurement of considerable angles with micrometric accuracy, that he resolved, when he should have the choice of a new telescope for the observatory, to secure some form of heliometer .

Nor is it difficult to imagine the probable course of reasoning which led Bessel to select the model of his new heliometer . Why, he might ask, should he not select the simple form of Dollond's first type ? Given the achromatic object-glass, why should not it be divided ? This construction would give all the See also:

• ADVANTAGE

advantage of the younger Dollond's object-glass micrometer, and more than its sharpness of See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition, without liability to the systematic errors which may be due to want of homogeneity of the object-glass; for the lenses will not be turned with respect to each other, but, in measurement, will always have the same relation in position angle to the See also:

• LINE

line joining the See also:

• OBJECTS

objects under observation . It is true that the scale will require to be capable of being read with much greater accuracy than i o1„bth of an See also:

• INCH (O. Eng. ynce from Lat. uncia, a twelfthpart; cf. " ounce," and see As)

inch—for that, even in a telescope of to ft. focus, would correspond with 2" of arc . But, after all, this is no See also:

• PRACTICAL

practical difficulty, for screws can be used to See also:

• SEPARATE

separate the lenses, and, by these screws, as in a See also:

• GASCOIGNE, GEORGE (c. 1535—1577)

Gascoigne micrometer, the separation of the lenses can be measured; or we can have scales for this purpose, read by microscopes, like the See also:

• TROUGHTON, EDWARD (1753-1835)

Troughton 1 circles of Piazzi or See also:

• POND
• POND, JOHN (c. 1767-1836)

Pond, or those of the See also:

• CAREY, HENRY (d. 1743)
• CAREY, HENRY CHARLES (1793-1879)
• CAREY, WILLIAM (1761-1834)

Carey circle, with almost any required accuracy . Whether Bessel communicated such a course of reasoning to See also:

• FRAUNHOFER, JOSEPH VON (1787-1826)

Fraunhofer, or whether that great artist arrived independently at like conclusions, we have been unable to ascertain with certainty . The fact remains that before 18202 Fraunhofer had completed one or more of the five heliometers (3 in. aperture and 39 in. focus) which have since become See also:

• HISTORICAL

historical instruments . In 1824 the great Konigsberg heliometer was commenced, and it was completed in 1829 . To sum up briefly the history of the development of the heliometer . The first application of the divided object-glass and the employment of double images in astronomical measures'is due to Savary in 1743 . To Bouguer in 1748 is due the true conception of measurement by double image without the auxiliary aid of a filar micrometer, viz. by changing the distance between two object-glasses of equal focus .

To Dollond in 1754 we owe the combination of Savary's See also:

• IDEA (Gr. Ibia, connected with i&eiv, to see; cf. Lat. species from specere, to look at)

idea of the divided object-glass with Bouguer's method of measurement, and the construction of the first really practical heliometers . To Fraunhofer, some time not long previous to 1820, is due, so far as we can ascertain, the construction of the first heliometer with an achromatic divided object-glass, i.e. the first heliometer of the modern type . The Modern Heliometer . The Konigsberg heliometer is represented in fig . 8 . No part of the equatorial mounting is shown in the figure, as it resembles in every respect the usual Fraunhofer mounting . An adapter h is fixed on a telescope-tube, made of See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

wood, in Fraunhofer's usual See also:

• FASHION (adapted from Fr. facon, agon, Lat. factio, making, facere, to do or make)

fashion . To this adapter is attached a See also:

• FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

flat circular flange h . The slides carrying the segments of the divided object-glass are mounted on a plate, which is fitted and ground to rotate smoothly on the flange h . Rotation is communicated by a pinion, turned by the handle c (concealed in the figure), which works in See also:

• TEETH (O.E. Eel); plural of tooth, O.E. top)

teeth cut on the edge of the flange h . The counterpoise w balances the head about its axis of rotation . The slides are moved by the screws a and b, the divided heads of which serve to measure the separation of the segments .

These screws are turned from the eye-end by bevelled wheels and pinions, the latter connected with the handles a', b' . The reading micrometers e, f also serve to measure, independently, the separation of the segments, by scales attached to the slides; such measurements can be employed as a check on those made by the screws . The measurement of position angles is provided for by'a graduated circle attached to the head . There is also a position circle, attached at in to the eye-end, provided with a slide to move the eye-piece radially from the axis of the telescope, and with a micrometer to measure the distance of an object from that axis . The See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring c, which carries the supports of the handles a', b', is capable of a certain amount of rotation on the tube . The See also:

• WEIGHT

weight of the handles and their supports is balanced by the counterpoise z . This ring is necessary in order to allow the rods to follow the micrometer heads when the position angle is changed . Complete rotation of the head is obviously impossible because of the interference of the See also:

• DECLINATION (from Lat. declinare, to decline)

declination axis with the rods, and therefore, in some angles, objects cannot be measured in two positions of the circle . The object-glass has an aperture of 61 in. and 102 in. focal length . There are three methods in which this heliometer can be used . First Method.—One of the segments is fixed in the axis of the telescope, and the eye-piece is also placed in the axis . Measures 1 The circles by See also:

• REICHENBACH
• REICHENBACH, GEORG VON (1772–1826)

Reichenbach, then almost exclusively used in See also:

• GERMANY
• GERMANY (Ger. Deutschland)

Germany, were read by verniers only .

2 The diameter of See also:

• VENUS

Venus was measured with one of these heliometers at the observatory of See also:

• BRESLAU (Polish Wraclaw)

Breslau by See also:

• BRANDES, GEORG MORRIS COHEN (1842– )

Brandes in 1820 (See also:

• BERLIN
• BERLIN, CONGRESS AND TREATY OF
• BERLIN, ISAIAH (1725–1799)

Berlin Jahrbuch, 1824, p . 164).are made with the moving segment displaced alternately on opposite sides of the fixed segment . Second Method.—One segment is fixed, and the measures are made as in the first method, excepting that the eye-piece is placed symmetrically with respect to the images under measurement . For this purpose the pos'tion angle of the eye-piece micrometer is set to that of the head, and the eye-piece is displaced from the axis of the tube (in the direction of the movable segment) by an amount equal to See also:

• HALF

half the angle under measurement . Third Method.—The eye-piece is fixed in the axis, and the segments are symmetrically displaced from the axis each by an amount equal to half the angle measured . Of these methods Bessel generally employed the first because of its simplicity, notwithstanding that it involved a resetting of the right See also:

• ASCENSION
• ASCENSION, FEAST OF THE

ascension and declination of the axis of the tube with each reversal of the segments . The See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief objections to the method are that, as one star is in the axis of the telescope and the other displaced from it, the images are not both in focus of the eye-piece,3 and the rays from the two stars do not make the same angle with the optical axis of each segment . Thus the two images under measurement are not defined with equal sharpness and symmetry . The second method is See also:

• FREE

free from the objection of non-coincidence in focus of the images, but is more troublesome in practice from the See also:

• NECESSITY (Lat. necessitas)

necessity for frequent readjustment of the position of the eye-piece . The third method is the most symmetrical of all, both in observation and reduction; but it was not employed by Bessel, on the ground that it involved the determination, of the errors of two screws instead of one . On the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand it is not necessary to reset the telescope after each reversal of the segments.¢ When Bessel ordered the Konigsberg heliometer, he was anxious to have the segments made to move in cylindrical slides, of which the radius should be equal to the focal length of the object-glass . Fraunhofer, however, did not execute this wish, on the ground that the See also:

• MECHANICAL

mechanical difficulties were too great .

M . L . G . Wichmann states (Konigsb . Beobach. See also:

• XXX

xxx . 4) that Besse] had indicated, by notes in his handbooks, the following points which should be kept in mind in the construction of future heliometers: (I) The segments should move in cylindrical slides;5 (2) the screw should be protected from dust;6 (3) the zero of the position circle should not be so liable to change; 7 (4) the distance of the optical centres of the segments should not change in different position angles or otherwise; 3 (5) the points of the micrometer screws should See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest on See also:

• IVORY (Fr. ivoire, Lat. ebur)
• IVORY, SIR JAMES (1765-1842)

ivory plates; 9 (6) there should be an apparatus for changing the See also:

• SCREEN (usually, but very doubtfully, connected with Lat. scrinium, a box for holding books, from scribere, to write; a connexion with Ger. Schranke, barrier, has been suggested)

screen,10 Wilhelm See also:

• STRUVE, FRIEDRICH GEORG WILHELM (1793-1864)

Struve, in describing the Pulkowa heliometer," made _ )-The distances of the optical centres of the segments from the eye-piece are in this method as 1; secant of the angle under measurement . In Bessel's heliometer this would amount to a difference of 1$ See also:

• BATH
• BATH, THOMAS THYNNE
• BATH, WILLIAM

bath of an inch when an angle of 1 ° is measured . For 2° the difference would amount to nearly --,lbth of an inch . Bessel confined his measures to distances considerably less than 1 ° . In criticizing Bessel's choice of methods, and considering the loss of time involved in each, it must be remembered that Fraunhofer provided no means of reading the screws or even the heads from the eye-end . Bessel's practice was to unclamp in declination, See also:

• LOWER

lower and read off the head, and then restore the telescope to its former declination reading, the clockwork meanwhile following the stars in right ascension . The setting of both lenses symmetrically would, under such circumstances, be very tedious .

5 This most important improvement would permit any two stars under measurement each to be viewed in the optical axis of each segment . The optical centres of the segments would also remain at the same distance from the eye-piece at all angles of separation . Thus, in measuring the largest as well as the smallest angles, the images of both stars would be equally symmetrical and equally well in focus . Modern heliometers made with cylindrical slides measure angles over 2°, the images remaining as See also:

• SHARP, GRANVILLE (1735-1813)
• SHARP, JAMES (1618-1679)
• SHARP, JOHN (1645-1714)
• SHARP, RICHARD (1759-1835)
• SHARP, WILLIAM (1749-1824)
• SHARP, WILLIAM (1856-1905)

sharp and perfect as when the smallest angles are measured . s Bessel found, in course of time, that the See also:

• ORIGINAL

original corrections for the errors of his screw were no longer applicable . He considered that the changes were due to See also:

• WEAR

wear, which would be much lessened if the screws were protected from dust . 7 The tube, being of wood, was probably liable to warp and twist in a very uncertain way . 8 We have been unable to find any published See also:

• DRAWING

drawing showing how the segments are fitted in their cells . - 9 We have been unable to ascertain the reasons which led Bessel to choose ivory planes for the end-See also:

• BEARINGS

bearings of his screws . He actually introduced them in the Konigsberg heliometer in 1840, and they were renewed in 1848 and 1850 . 1s A screen of wire See also:

• GAUZE

gauze, placed in front of the segment through which the fainter star is viewed, was employed by Bessel to equalize the brilliancy of the images under observation . An arrangement, afterwards described, has been fitted in modern heliometers for placing the screen in front of either segment by a handle at the See also:

• EVE

eve-end .

11 This heliometer resembles Bessel's, except that its See also:

• FOOT

foot is a solid See also:

• BLOCK
• BLOCK (from the Fr. bloc, and possibly connected with an Old Ger. Block, obstruction, cf. " baulk ")
• BLOCK, MAURICE (1816–19or)

block of See also:

• GRANITE (adapted from the Ital. granite, grained; Lat. granum, grain)

granite instead of the See also:

• ILL

ill-conceived wooden structure that supported his instrument . The object-glass is of 7.4 in. aperture and 123 in. focus . by Merz in 1839 on the model of Bessel's heliometer, submits the following suggestions for its improvement:" (1) to give automatic-ally to the two segments simultaneous equal and opposite movement ; 2 and (2) to make the tube of See also:

• METAL
• METAL (through Fr. from Lat. metallum, mine, quarry, adapted from Gr. µATaXAov, in the same sense, probably connected with ,ueraAAdv, to search after, explore, µeTa, after, aAAos, other)

metal instead of wood ; to attach the heliometer head firmly to this tube; to See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place the eye-piece permanently in the axis of the telescope; and to See also:

• FIX, THEODORE (180o-1846)

fix a strong See also:

• CRADLE (of uncertain etymology, possibly connected with " crate " and " creel," i.e. basket; the derivation from a Celtic word, with a sense of rocking, is scouted by the New English Dictionary)

cradle on the end of the declination axis, in which the tube, with the attached head and eye-piece, could rotate on its axis . Both suggestions are important . The first is originally the idea of Dollond ; its advantages were overlooked by his son, and it seems to have been quite forgotten till resuggested by Struve . But the method is not available if the separation is to be measured by screws; it is found, in that See also:

• CASE
• CASE, JOHN (d. 1600)

case, that the direction of the final See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion of turning of the screw must always be such as to produce motion of the segment against gravity, otherwise the " loss of time " is See also:

• APT

apt to be variable . Thus the simple connexion of the two screws by See also:

• COG

cog-wheels to give them automatic opposite motion is not an available method unless the separation of the segments is independently measured by scales . Struve's second suggestion has been adopted in nearly all succeeding heliometers . It permits complete rotation of the tube and measurement of all angles in reversed positions of the circle; the handles that move the slides' can be brought down to the eye-end, inside the tube, and consequently made to rotate with it; and the position circle may be placed at the end of the cradle next the eye-end where it is convenient of See also:

• ACCESS (Lat. accessus)

access . Struve also points out that by attaching a fine scale to the focusing slide of the eye-piece, and knowing the coefficient of expansion of the metal tube, the means would be provided for determining the See also:

• ABSOLUTE (Lat. absolvere, to loose, set free)

absolute change of the focal length of the object-glass at any time by the simple See also:

• PROCESS

process of focusing on a double star . This, with a knowledge of the temperature of the screw or scale and its coefficient of expansion, would enable the change of screw-value to be determined at any instant . It is probable that the See also:

• BONN

Bonn heliometer was in course of construction before these suggestions of Struve were published or discussed, since its construction resembles that of the Konigsberg and Pulkowa instruments .

Its dimensions are similar to those of the former instrument . Bessel, having been consulted by the celebrated statesman, See also:

• SIR

Sir See also:

• ROBERT
• ROBERT (1275—1343)
• ROBERT, HUBERT (1753-1808)
• ROBERT, LOUIS LEOPOLD (1794-1835)

Robert See also:

• PEEL
• PEEL, ARTHUR WELLESLEY PEEL, 1ST VISCOUNT (1829- )
• PEEL, SIR ROBERT, BART

Peel, on behalf of the See also:

• RADCLIFFE
• RADCLIFFE, ANN (1764-1823)
• RADCLIFFE, JOHN (1650-1714)
• RADCLIFFE, SIR GEORGE (1593-1657)

Radcliffe trustees, as to what instrument, added to the Radcliffe Observatory, would probably most promote the See also:

• ADVANCEMENT

advancement of astronomy, strongly advised the selection of a heliometer . The order for the instrument was given to the Repsolds in 184o, but " various circumstances, for which the makers are not responsible, contributed to delay the completion of the instrument, which was not delivered before the See also:

• WINTER, JOHN STRANGE
• WINTER, PETER (c. 1755-1825)

winter of 1848." The See also:

• BUILDING

building to receive it was commenced in See also:

• MARCH
• MARCH (1) (from Fr. marcher, to walk; the earliest sense in French appears to be " to trample," and the origin has usually been found in the Lat. marcus, hammer; Low Lat. marcare, to hammer; hence to beat the road with the regular tread of a soldier: cf.
• MARCH, AUZIAS (c. "1395-1458)
• MARCH, EARLS OF
• MARCH, FRANCIS ANDREW (1825– )

March 1849 and completed in the end of the same year . This instrument has a superb object-glass of 71 in. aperture and 126 in. focal length . The makers availed themselves of Bessel's suggestion to make the segments move in cylindrical slides, and of Struve's to have the head attached to a brass tube; the eye-piece is set permanently in the axis, and the whole rotates in a cradle attached to the declination axis . They provided a splendid, rigidly mounted, equatorial stand, fitted with every luxury in the way of slow motion, and scales for measuring the displacement of the segments were read by powerful micro-meters from the eye-end.' It is somewhat curious that, though Struve's second suggestion was adopted, his first was overlooked by the makers . But it is still more curious that it was not afterwards carried out, for the communication of automatic symmetrical motion to both segments only involves a simple alteration previously described . But, as it came from the hands of the makers in 1849, the See also:

• OXFORD
• OXFORD, EARLS OF
• OXFORD, EDWARD DE VERE, 17TH EARL
• OXFORD, JOHN DE VERE, 13TH EARL OF (1443-1513)
• OXFORD, PROVISIONS OF
• OXFORD, ROBERT DE VERE, 9TH EARL OF (1362-1392)
• OXFORD, ROBERT HARLEY, 1ST

Oxford heliometer was incomparably the most powerful and perfect instrument in the See also:

• WORLD

world for the highest order of micrometric See also:

• RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

research . It so remained, unrivalled in every respect, till 1873 . As the transit of Venus of 1874 approached, prepara- tions were set on foot by the See also:

• GERMAN
• GERMAN, DUTCH AND SCANDINAVIAN

German See also:

• GOVERNMENT
• GOVERNMENT (0. Fr. governement, mod. gouvernement, O. Fr. governer, mod. gouverner, from Lat. gubernare, to steer a ship, guide, rule; cf. Gr. xv(3epvav)

Government in good time; a See also:

• COMMISSION (from Lat. commissio, committere)

commission of the most celebrated astronomers was appointed, and it was resolved that the heliometer should be the instrument chiefly relied on . The four long-neglected small heliometers made by Fraunhofer were brought into requisition . Fundamental alterations were made upon them : their wooden tubes were replaced by tubes of metal; r Description de l'observatoire central de Pulkowa, p .

208 . ' Steinheil applied such motion to a double-image micrometer made for Struve . This instrument suggested to Struve the above-mentioned idea of employing a similar motion for the heliometer . ' See also:

• MANUEL, EUGENE (1823–1901)
• MANUEL, JACQUES ANTOINE (1775-1827)
• MANUEL, LOUIS PIERRE (1751-1793)

Manuel See also:

• JOHNSON, ANDREW
• JOHNSON, ANDREW (1808–1875)
• JOHNSON, BENJAMIN (c. 1665-1742)
• JOHNSON, EASTMAN (1824–1906)
• JOHNSON, REVERDY (1796–1876)
• JOHNSON, RICHARD (1573–1659 ?)
• JOHNSON, RICHARD MENTOR (1781–1850)
• JOHNSON, SAMUEL (1709-1784)
• JOHNSON, SIR THOMAS (1664-1729)
• JOHNSON, SIR WILLIAM (1715–1774)
• JOHNSON, THOMAS

Johnson, M.A., Radcliffe observer, Astronomical Observations made at the Radcliffe Observatory, Oxford, in the Year z85o, Introduction, p. iii . ' The See also:

• ILLUMINATION

illumination of these scales is interesting as being the first application of See also:

• ELECTRICITY

electricity to the illumination of astronomical instruments . Thin See also:

• PLATINUM [symbol Pt, atomic weight 145.0 (0=16)]

platinum wire was rendered incandescent by a voltaic current; a small incandescent electric See also:

• LAMP (from Gr. Xag ras, a torch, Xaµaav, to shine)

lamp would now be found more satisfactory.means of measuring the focal point were provided; symmetrical motion was given to the slides; scales on each slide were provided instead of screws for measuring the separation of the segments, and both scales were read by the same micrometer See also:

• MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope ; a metallic thermometer was added to determine the temperature of the scales . These small instruments have since done admirable work in the hands of Schur, See also:

• HAT

Hat-twig, Kiistner, Elkin, Auwers and others . The See also:

• RUSSIAN

Russian Government ordered three new heliometers (each of 4 in. aperture and 5 ft. focal length) from the Repsolds, and the See also:

• DESIGN (Fr. desiin, drawing; Lat. designare, to mark out)

design for their construction was superintended by Struve, Auwers and Winnecke, the last-named making the necessary experiments at Carlsruhe . Fig . 9 represents the resulting type of instrument which was finally designed and constructed by Repsolds . The brass tube, strengthened at the bearing points by strong truly turned collars, rotates in the See also:

• CAST (from the verb meaning " to throw "; the word is Scand. in origin, cf. Dan. kaste, and Swed. kasta; " cast " in Middle Eng. took the place of the A.S. weor pan, cf. Ger. werf en)

cast See also:

• IRON [symbol Fe, atomic weight 55.85 (0=16)]

iron cradle q attached to the declination axis. a is the eye-piece fixed in the optical axis, b the micrometer for reading both scales. c and d are telescopes for reading the position circle p, e the handle for See also:

• QUICK

quick motion in position angle, f the slow motion in position angle, g the handle for changing the separation of the segments by acting on the See also:

• BEVEL (from an O. Fr. word, cf. mod. biveau, a joiner's instrument)

bevel-See also:

• WHEEL (0. Eng. hweol, hweohl, &c., cognate with Icel. hjol, Dan. hiul, the Indo-European root is seen in Sanskrit chakra, Gr. r in Aos, circle, whence " cycle ")
• WHEEL, BREAKING ON THE

wheel g' (fig. to). h is a milled head connected by a See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

rod with h' (fig . 1o), for the purpose of inter-posing at See also:

• PLEASURE (through Fr. plaisir from Lat. placere, to please; Gr. rlbovii)

pleasure the See also:

• PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

prism 7r in the axis of the reading micrometer; this enables the observer to view the graduations on the See also:

• FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face of the metallic thermometer rr (composed of a rod of brass and a rod of See also:

• ZINC

zinc). i is a milled head connected with the wheel i'i' (fig. to), and affords the means of placing the screen s (fig .

9), counterpoised by w over either half of the object-glass. k clamps the telescope in declination, n clamps it in right ascension, and the handles m and l provide slow motion in declination and right ascension respectively . The details of the interior mechanism of the " head " will be almost w C FIG . Io . evident from fig. to without description . The screw, turned by the wheels at g', acts in a toothed arc, whence, as shown in the figure, equal and opposite motion is communicated to the slides by the jointed rods v, v . The slides are kept firmly down to their bearings by the rollers r, r, r, r, attached to axes which are, in the See also:

• MIDDLE

middle, very strong springs . Side-shake is prevented by the screws and, pieces k, k, k, k . The scales are at it, n; they are fastened only at the middle, and are kept down by the brass pieces t, t . A similar heliometer was made by the Repsolds to the order of See also:

• LORD
• LORD (O. Eng. hldford, i.e. hldfweard, the warder or keeper of bread, hlIf, loaf; the word is not represented in any other Teutonic language)
• LORD, JOHN (1810-1894)

Lord See also:

• LINDSAY

Lindsay for his See also:

• MAURITIUS

Mauritius expedition in 1874 . It differed only from the three Russian instruments in having a mounting by the Cookes in which the declination circle reads from the eye-end.' This instrument was afterwards most generously See also:

• LENT (0. Eng. lenclen, " spring," M. Eng. lenten, lente, lent; cf. Dut. lente, Ger. Lenz, " spring," 0. H. Ger. lenzin, lengizin, lenzo, probably from the same root as " long " and referring to " the lengthening days ")

lent by Lord Lindsay to Gill for his expedition to Ascension in 1877.6 These four See also:

• REPSOLD, JOHANN GEORG (1771–1830)

Repsold heliometers proved to be excellent instruments, ' For a detailed description of this instrument see Dunecht Publications, vol. ii . 6 Mem . Royal Astronomical Society, xlvi 1-172 .

v A --J .. -- ,ijllilll~'~lh thus memo, AN . I r See also:

• FIR

fir, n r See also:

• SPA

spa or i• ;i s easy and convenient in use, and yielding results of very high accuracy in measuring distances . Their slow motion in position angle, how-ever, was not all that could be desired . When small movements were communicated to the handle e (fig . 9) by the tangent screw f, acting on a small toothed wheel clamped to the rod connected with the See also:

• DRIVING (from " to drive," i.e. generally to propel, force along or in, a word common in various forms to the Teutonic languages)

driving pinion, there was apt to be a torsion of the rod rather than an immediate See also:

• ACTION

action . Thus the slow motion would take place the observer . This alteration and the new equatorial mounting have been admirably made by Grubb; the result is completely successful . The instrument so altered was in use at the Cape Observatory from March 1881 till 1887 in deter-See also:

• MINING

mining the See also:

• PARALLAX (Gr. aapa?X6, alternately)

parallax of some of the more interesting See also:

• SOUTHERN

southern stars . The instrument then passed, by See also:

• PURCHASE

purchase from Gill, to Lord McLaren, by whom it was presented to the Royal Observatory, See also:

• EDINBURGH

Edinburgh . Still more recently the Repsolds have completed a new heliometer for Yale See also:

• COLLEGE (Collegium)

College, New Haven, See also:

• UNITED

United States . The object-glass is of 6 in. aper- See also:

• TURE

ture and 98 in. focal length .

The mounting, the tube, See also:

• OBJECTIVE, or OBJECT GLASS

objective-See also:

• CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell, slides, &c., are all of See also:

• STEEL, FLORA ANNIE (1847– )

steel.' The instrument is shown in fig . 11 . The circles for position angle and declination are read by micrometer-microscopes illuminated by the lamp L; the scales are illuminated by the lamp 1 . T is part of the tube proper, and turns with the head . The tube V, on the contrary, is attached to the cradle, and merely forms a support for the finder Q, the handles at f and p, and the moving ring P . The latter gives quick motion in position angle; the handles at p clamp and give slow motion in position angle, those at f clamp and give slow motion in right ascension and declination. a is the eye-piece, b the handle for moving the seg- ments, c the micrometer microscope for reading the scales and scale micrometer, d the micrometer readers of the position and declination circles, e the handle for rotating the large wheel E which carries the screens . The See also:

• HOUR

hour circle is also read by microscopes, and the instrument can be used in both positions (tube preceding and following) for elimination of the effect of flexure on the position angles . Elkin found that the chief drawbacks to See also:

• SPEED, JOHN (1552–1629)

speed and convenience in working this heliometer were: (I) The loss of time involved in entering the correspond- See also:

• ING

ing readings of the micrometer pointings on two scales . (2) That an additional motion inter- mediate between the quick and slow motion in position angle was necessary, because, whilst the slow motion provided by Repsolds was admirably adapted for adjusting the pointings in position angle, it was too slow for causing the images to "See also:

• CROSS

cross through " each other in the process of measur- ing distances . To remedy See also:

• DRAWBACK

drawback (I) Repsolds devised the form of See also:

• PRINTING (from Lat. imprimere, O. Fr. empreindre)

printing micrometer which is shown in See also:

• FIGS

figs . 12 and 13 . This micrometer is provided with two pairs of parallel webs .

One fixed pair of webs is attached to the micrometer-See also:

• BOX
• BOX (Gr. irl or, Lat. buxus, box-wood; cf. 2r6Eir, a pyx)

box, the other pair is moved by the screw S . The whole micrometer-box is moved by by jerks instead of with the necessary smoothness and certainty . When the heliometer-part of Lord Lindsay's heliometer was acquired by Gill in 1879, he changed the manner of imparting the motion in question . A square toothed racked wheel was applied to the tube at r (fig . 9) . This wheel is acted on by a tangent screw whose bearings are attached to the cradle; the screw is turned by means of a handle supported by bearings attached to the cradle, and coming within convenient reach of the observer's hand . The tube turns smoothly in the racked wheel, or can be clamped to it at the will of the screw attached to the heads . Accordingly, in reading the scales A and B (attached to the slides which carry the two halves of the object-glass), it is only necessary to turn the screws until the fixed I The See also:

• PRIMARY

primary object was to have the object-glass mounted in steel cells, which more nearly correspond in expansion with glass . It became then desirable to make the head of steel for See also:

• SAKE

sake of uniformity of material, and the advantages of steel in lightness and rigidity for the tube then became evident . double See also:

• WEB (a word common to Teutonic languages, cf. Du. webbe, Dan. vaev, Ger. Gewebe, all from the Teutonic wabh—to weave)

web is pointed symmetrically on one of the divisions of scale A, then to move the other double web by the screw S until it is symmetrically pointed on the adjoining division of scale B . By turning the quick acting screw P (fig . 13) to the right, the See also:

• CUSHION (from O. Fr. coisson, coussin; according to the New English Dict., from Lat. coxa, a hip; others say from Lat. culcita, a quilt)

cushion C (which is faced with See also:

• INDIA
• INDIA, FRENCH

India-See also:

• RUBBER, INDIARUBBER

rubber) presses the paper ribbon (shown in fig .

13) against the index-edge and type-wheels, and thus the beautifully cut divisions of the micrometer-head, the See also:

• NUMBERS, BOOK OF
• NUMBERS, PARTITION OF

numbers marking the 1~a parts of the head, the index and the See also:

• TOTAL

total number of revolutions are all sharply embossed together upon the 3 :38 paper ribbon . Fig . 14 shows the See also:

• RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

record of several successive paintings on the same scale as that given by the micrometer . The See also:

• REVERSE

reverse motion of P automatically moves the paper ribbon forward, ready to receive the next impression . It must be mentioned a- 2e that the pressure of the cushion C on the type-wheels has no See also:

• INFLUENCE (Late Lat. influentia, from influere, to flow in)

influence whatever upon the micrometer-screw, because the type-wheels are mounted on a hollow cylindrical axis, concentric with the axis of the screw, but entirely disconnected from the screw itself . The only connexion between the type-wheel and the screw-head S is by the See also:

• PIN (a doublet with " pen " from Lat. pinna, feather, pinnacle, which is said to contain the same root as irirvs, pine tree, and properly to mean a sharp point or end)

pin p (which is screwed into S), the cylindrical end of which acts in a slot cut in the type-wheel . To remedy drawback (2) Repsolds provided See also:

• F1G

F1G . 14. for the Yale heliometer an additional handle for motion in position angle, intermediate in velocity between the original quick and slow motions . In the 7-in. heliometer, completed in 1887 for the Royal Observatory at the Cape of Good Hope, Repsolds, on Gill's suggestion, introduced the following improvements: (a) Four different speeds of motion in position angle were provided . The quickest movement is given by the hand-ring, 73 (fig . 15) . This ring runs between See also:

• FRICTION (from Lat. fricare, to rub)

friction wheels and is provided with teeth on its inner periphery, and these teeth transmit motion to a pinion on a spindle having at its other end another pinion which, through an intermediate wheel, rotates the heliometer tube .

The transmission spindle, just mentioned, carries at its end a head, 74, which, if turned directly, gives the second speed . The slowest speed is given by means of a tangent screw which is carried by a See also:

• BALL (in Mid. Eng. bal; the word is probably cognate with " bale," Teutonic in origin, cf. also Lat. falls, and Gr. 7raXXa)
• BALL, JOHN (1585-1640)
• BALL, JOHN (1818-1889)
• BALL, JOHN (d. 1381)
• BALL, SIR ALEXANDER JOHN, BART
• BALL, THOMAS (1819- )

ball-bearing on the flange of the telescope- From See also:

• ENGINEERING

Engineering, vol. alix . FIG . 15 . See also:

• SLEEVE (O. Eng. slieve, slyf, a word allied to " slip," cf. Dutch sloof, apron)

sleeve, whilst its See also:

• NUT
• NUT (0. Eng. knutu, cf. Dutch noot, Ger. Nuss; allied with Gael. cno; it is not of the same form as Lat. nux)

nut is double-jointed to a ring that encircles the flange of the heliometer-tube . This ring is provided with a clamping screw, which, through the intervention of bevel-See also:

• GEAR (connected with " garb," properly elegance, fashion, especially of dress, and with " gar," to cause to do, only found in Scottish and northern dialects; the root of the word is seen in the Old Tent. garwjan, to make ready)

gear and rods, is operated by means of the hand-wheel 78 . With similar bevel-gear and rods the tangent screw is connected to the hand-wheel, i9, by which the observer communicates the See also:

• FOURTH

fourth or slowest motion in position angle . Finally the hand-wheel 8o is connected by gearing to the rod carrying the hand-wheel 79, and it can thus be used to give the latter a more rapid motion than if used See also:

• DIRECT

direct; this constitutes the third speed of movement . (b) In lieu of oil-lamps, small, conveniently placed incandescent electric 6-volt lamps are employed; and these are fitted with suitable switches and variable resistances . Thus the scales, the position- and declination-circles, the See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

field of view, the heads of all the micrometer-microscopes, the focusing scale, &c., are read without the aid of a hand-lamp and with an amount of illumination that can be regulated at the observer's pleasure, (c) A See also:

• BUTTON (Fr. bouton, O. Fr. boton, apparently from the same root as bouter, to push)

button in the centre of the position-angle handle (74) connects with a See also:

• CHRONOGRAPH

chronograph which enables the observer to record the instant of observation . Little card-holders (8i) (also illuminated) enable the astronomer to enter beforehand the R.A. and Dec. of the object to be observed, the scale divisions to be pointed upon, and thus, in measures of distance, with the aid of the chronograph and printing micrometer, enable the observer to adjust the instrument for observation and obtain a record of his observations without the aid of a hand-lamp or the necessity to make any records in his notebook . In observations of position angle one of the two tablets 81 can be used to record the readings .

(d) The scales are made of iridio-platinum instead of See also:

• SILVER