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Originally appearing in Volume V26, Page 568 of the 1911 Encyclopedia Britannica.
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INSTRUMENTS, &C. We proceed to give an account of the methods and principles of construction of the various kinds of telescopes, and Ayscough was an optician in Ludgate Hill, London. to describe in detail special typical instruments, which, owing to the work accomplished by their aid or the practical advances exemplified in their construction, appear most worthy of record or study. Refracting Telescope In its simplest form the telescope consists of a convex objective capable of forming an image of a distant object and of an eye-lens, concave or convex, by which the image so formed is magnified. When the axis of the eye-lens coincides with that of the object-glass, and the focal point of the eye-lens is coincident with the principal focus of the object-lens, parallel rays incident upon the object-glass will emerge from the eye-piece as parallel rays. These, falling in turn on the lens of the human eye, are converged by it and form an image on the retina. Fig. I shows the course of the rays when the eye-lens is convex (or positive), fig. 2 when the eye-lens is concave (or negative). The former represents Kepler's, the latter Lippershey's or the Galilean telescope. The magnifying power obviously depends on the proportion of the focal length of the object-lens to that of the eye-lens, that is, magnifying power=F/e, where F is the focal length of the object-lens and e that of the eye-lens. Also the diameter of the pencil or parallel rays emerging from the eye-lens is to the diameter of the object-lens inversely as the magnifying power of the telescope. Hence one of the best methods of determining the magnifying power of a telescope is to measure the diameter of the emergent pencil of rays, after the telescope has been adjusted to focus upon a star, and to divide the diameter of the object-glass by the diameter of the emergent pencil. If we desire to utilize all the parallel rays which fall upon an object-glass it is necessary that the full pencil of emerging rays should enter the observer's eye. Assuming with Sir William Herschel that the normal pupil of the eye distends to one-fifth of an inch in diameter when viewing faint objects, we obtain the rule that the minimum magnifying power which can be efficiently employed is five times the diameter of the object-glass expressed in inches.' The defects of the Galilean and Kepler telescopes are due to the chromatic and spherical aberration of the simple lenses of which they are composed. The substitution of a positive or negative eye-piece for the simple convex or concave eye-lens, and of an achromatic object-glass for the simple object-lens, transforms these early forms into the modern achromatic telescope. The Galilean telescope with a concave eye-lens instead of an eye-piece still survives as the modern opera-glass, on account of its shorter length, but the object-glass and eye-lens are achromatic combinations. (D. GI.) Telescope Objectives.3-In spite of the improvements in the manufacture of optical glass (see GLASS) practically the same crown and flint glasses as used by John Dollond in 1758 for achromatic objectives are still used for all the largest of the modern refracting telescopes. It has long been known that the spectra of white or solar light yielded by ordinary crown and flint glasses are different: that while two prisms of such glasses may be arranged to give exactly the same angular dispersion between two Fraunhofer 2 In the case of short-sighted persons the image for very distant objects (that is, for parallel rays) is formed in front of the retina; therefore, to enable such persons to see distinctly, the rays emerging from the eye-piece must be slightly divergent; that is, they must enter the eye as if they proceeded from a comparatively near object. For normal eyes the natural adaptation is not to focus for quite parallel rays, but on objects at a moderate distance, and practically, therefore, most persons do adjust the focus of a telescope, for most distinct and easy vision, so that the rays emerge from the eye-piece very slightly divergent. Abnormally short-sighted per-sons require to push in the eye-lens nearer to the object-glass, and long-sighted persons to withdraw it from the adjustment employed by those of normal sight. It is usual, however, in computations of the magnifying power of telescopes, for the rays emerging from the eye-piece when adjusted for distinct vision to be parallel. 2 For the methods of grinding, polishing and testing lenses, see OBJECTIVE. lines, such as C and F, yet the flint glass prism will show a relative drawing out of the blue end and a crowding together of the red end of the spectrum, while the crown prism shows an opposite tendency. This want of proportion in the dispersion for different regions of the spectrum is called the " irrationality of dispersion "; and it is as a direct consequence of this irrationality, that there exists a secondary spectrum or residual colour dispersion, showing itself at the focus of all such telescopes, and roughly in proportion to their size. These glasses, however, still hold the field, although glasses are now produced whose irrationality of dispersion has been reduced to a very slight amount. The primary reason for this retention is that nothing approaching the difference in dispersive power between ordinary crown glass and ordinary dense flint glass (a difference of 1 to II) has yet been obtained between any pair of the newer glasses. Consequently, for a certain focal length, much deeper curves must be resorted to if the new glasses are to be employed; this means not only greater difficulties in workmanship, but also greater thickness of glass, which militates against the chance of obtaining large disks quite free from striae and perfect in their state of annealing. In fact, superfine disks of over 15 in. aperture are scarcely possible in most of the newer telescope glasses. Moreover the greater depths of the curves (or " curvature powers ") in itself neutralize more or less the advantages obtained from the reduced irrationality of dispersion. When all is taken into consideration it is scarcely possible to reduce the secondary colour aberration at the focus of such a double object-glass to less than a fourth part of that prevailing at the focus of a double objective of the same aperture and focus, but made of the ordinary crown and flint glasses. The only way in which the secondary spectrum can be reduced still further is by the employment of three lenses of three different sorts of glass, by which arrangement the secondary spectrum has been reduced in the case of the Cooke photo visual objective to about 1/loth part of the usual amount, if the whole region of the visible spectrum is taken into account. It is possible to construct a triple objective of two positive lenses enclosing between them one negative lens, the two former being made of the same glass. For relatively short focal lengths a triple construction such as this is almost necessary in order to obtain an objective free from aberration of the 3rd order, and it might be thought at first that, given the closest attainable degree of rationality between the colour dispersions of the two glasses employed, which we will call crown and flint, it would be impossible to devise another form of triple objective, by retaining the same flint glass, but adopting two sorts of crown instead of only one, which would have its secondary spectrum very much further reduced. Yet such is the rather surprising fact. But it can be well illustrated in the case of the older glasses, as the following case will show. The figures given are the partial dispersions for ordinary crown and ordinary extra dense flint glasses, styled in Messrs Schott's catalogue of optical glasses as o•6o and 0.102 respectively, having refractive indices of 1.5179 and 1.6489 for the D ray respectively, and (µDr)/(µF--µc)=6o•2 and 33'8 respectively to indicate their dispersive powers (inverted), =v. C to F A to D D to F F to G * * * * o•6o •oo86o I•oo0 .00533 '643 '00605 •703 .00487 •566 0.102 .01919 1.000 .01152 •600 .01372 •714 •01180 .615 I .02779 ~ •613 I .7111•o1667 •600 1.000.01685 •01977 The Aµ from C to F being taken as unity in each case, then the c1 µ's for the other regions of the spectrum are expressed in fractions (C to F) and are given under the asterisks. Let it be supposed that two positive lenses of equal curvature powers are made out of these two glasses, then in order to represent the combined dispersion of the two together the two 0µ's for each spectral region may be added together to form A'µ as in the line below, and then, on again expressing the partial t1'µin terms of D'µ (C to F) we get the new figures in the bottom row beneath the asterisks. We find that we have now got a course of dispersion or degree of rationality which very closely corresponds to that of an ordinary light flint glass, styled 0.569 in Schott's catalogue, and having µD 1.5738 and (µD-1)/(µF-µc) =41'4=v, the figures of whose course of dispersion are as below:- Light Flint Glass 0.569. Hence it is clear that if the two positive lenses of equal curvature power of o•6o and 0•IO2 respectively are combined with a negative lens of light flint o•569, then a triple objective, having no secondary spectrum (at any rate with respect to the blue rays), may be obtained. But while an achromatic combination of o•6o and o•102 alone will yield an objective whose focal length is only 1.28 times the focal length of the negative or extra dense flint lens, the triple combination will be found to yield an objective whose focal length is 73 times as great as the focal length of the negative light flint lens. Hence impossibly deep curvatures would be required for such a triple objective of any normal focal length. This case well illustrates the much closer approach to strict rationality of dispersion which is obtainable by using two different sorts of glass for the two positive lenses, even when one of them has a higher dispersive power than the glass used for the negative lens. It is largely to this principle that the Cooke photo visual objective of three lenses (fig. 3) owes its high degree of achromatism. This form of objective has been successfully made up to 122 in. clear aperture. The front lens is made of baryta light flint glass (o.543 of Schott's catalogue) and the back lens of a crown glass, styled 0.374 in Schott's older lists. The table gives their partial dispersions for six different regions of the spectrum also expressed (in brackets below) as fractional parts of the dispersion from C to F. C to F IAtoC DtoF'EtoFFtoG' FtoH 0.543 .01115 .00374 .Q0790 •00369 •00650 •01322 PD = 1 .564 (1.0000) (.3354) (.7085) (.3309) (.5830) (1.1857) v = 50. 7 0'374 •00844 •00296 .00593 .00274 .00479 .00976 µD = 1.511 ( 1 .0000);(.3507) (.7026) (.3247) (.5675) (1.1564) v T6o•8 Since the curvature powers of the positive lenses are equal, the partial dispersions of the two glasses may be simply added together, and we then have:- [0'543+0'3741 CtoF AtoC DtoF EtoF FtoG' FtoH .01959 •00670 .01383 .00643 .01129 .02298 (1.0000) (.3420) (.7059) (.3282) (.5763) (1.1730) The proportions given on the lower line may now be compared with the corresponding proportional dispersions for borosilicate flint glass 0.658, closely resembling the type 0.164 of Schott's list, viz.:- [o.658 (ND =1.546) v=50.11 CtoF Ato C 1.0000 •3425 CtoF A' to D D to F F to G .01385 I•000 .00583 .615 •00987 .713 .00831 •600 D to F E to F F to G' F to H •7052 .3278 .5767 1'1745 A slight increase in the relative power of the first lens of 0.543 would bring about a still closer correspondence in the rationality, but with the curves required to produce an object-glass of this type of 6 in. aperture and Io8 in. focal length a discrepancy of 1 unit in the 3rd decimal place in the above proportional figures would cause a linear error in the focus for that colour of only about .025 in., so that the largest deviation implied by the tables would be a focus for the extreme violet H ray about •037 longer than the normal. It will be seen, then, that the visual and photographic foci are now merged in one, and the image is practically as achromatic as that yielded by a reflector. Other types of triple object-glasses with reduced secondary spectra have recently been introduced. The extension of the image away from the axis or size of field available for covering a photographic plate with fair definition is a function in the first place of the ratio between focal length and aperture, the longer focus having the greater relative or angular covering power, and in the second a function of the curvatures of the lenses, in the sense that the objective must be free from coma at the foci of oblique pencils or must fulfil the sine condition (see ABERRATION). Eye-pieces.—The eye-pieces or oculars through which, in case of visual observations, the primary images formed by the objective are viewed, are of quite secondary importance as regards definition in the central portion of the field of view. If an eye-piece blurs the definition in any degree in the centre of the field it must be very badly figured indeed, but the definition towards the edge of the field, say at 20° away from the centre of the apparent field of view, depends very intimately upon the construction of the eye-piece. It must be so designed as to give as flat an image as is possible consistently with freedom from astigmatism of oblique pencils. The mere size of the apparent field of view depends upon obtaining the oblique pencils of light emerging from it to cross the axis at the great possible angle, and to this end the presence of a field-lens is indispensable, which is separated from the eye-lens by a considerable interval. The earlier arrangement of two lenses of the Huygenian eye-piece (see MICROSCOPE) having foci with ratio of 3 to I, gives a fairly large flat field of view approximately free from distortion of tangential lines and from coma, while the Mittenzwey variety of it (fig. 4) in which the field-lens is changed into a meniscus having radii in about the ratio of +I to – 9 gives still better results, but still not quite so good as the results obtained by using the combination of two convexo-plane lenses of the focal ratio 2 to I. In the Ramsden eye-piece (see MICROSCOPE) the focal lengths of the two piano-convex lenses are equal, and their convexities are turned towards one another. The field-lens is thus in the principal focal plane of the eye-lens, if the separation be equal to 2( This is such a practical drawback that the separation is generally ;ths or gths of the theoretical, and then the primary image viewed by the eye- piece may be rather outside the field-lens, which is a great practical advantage, especially when a reticule has to be mounted in the primary focal plane, although the edge of the field is not quite achromatic under these conditions. Kellner Eye-piece.—In order to secure the advantage of the principal focal plane of the eye-piece being well outside of the field-lens and at the same time to obtain a large flat field of563 view with oblique achromatism and freedom from coma and distortion, there is no better construction than the modified Kellner eye-piece (fig. 5) such as is generally used for prismatic binoculars. ft consists of a piano-convex field-lens of crown. glass and an approximately achromatic eye-lens, some distance behind it, consisting of an equi-convex crown lens cemented to a concavoplane flint lens, the latter being next to the eye. There are also other eye-pieces having the field-lens double or achromatic as well as the eye-lens. In cases where it is important to get the maximum quantity of light into the eye, the field-lens is discarded and an achromatic eye-lens alone employed. This yields a very much smaller field of view, but it is very valuable for viewing feeble telescopic objects and very delicate planetary or lunar details. Zeiss and Steinheil's monocentric eye-pieces and the Cooke single achromatic eye-piece (fig. 6) are examples of this class of oculars. (H. D. T.) Reflecting Telescope. The following are the various forms of reflecting telescopes: The Gregorian telescope is represented in fig. 7, A A and B B are concave mirrors having a common axis and their concavities facing each other. The focus of A for parallel rays is at F, that of B for parallel rays at f—between B and F. Parallel rays falling on A A converge at F, where an image r. is formed; the rays are then reflected from B and converge at P, where a second and more enlarged image is. formed. Gregory himself showed that, if the large mirror were a segment of a paraboloid of revolution whose focus is F, and the small mirror an ellipsoid of revolution whose foci are F and P respectively, the resulting image will be plane and undistorted. The image formed at P is viewed through the eye-piece at E, which may be of the Huygenian or Ramsden type. The focal adjustment is accomplished by the screw S, which acts on a slide carrying an arm to which the mirror B is attached. The practical difficulty of constructing Gregorian telescopes of good defining quality is very considerable, because if spherical mirrors are employed their aberrations tend to increase each other, and it is extremely difficult to give a true elliptic figure to the necessarily deep concavity of the small speculum. Short appears to have systematically conquered this difficulty, and his Gregorian telescopes attained great celebrity. The use. of the Gregorian form is, however, practically abandoned in the present day. The magnifying power of the telescope is =Ff/ex, where F and f are respectively the focal lengths of the large and the small mirror, e the focal length of the eye-piece, and x the distance between the principal foci of the two mirrors (=Ff in the diagram) when the instrument is in adjustment for viewing distant objects. The images are erect. The Cassegrain telescope differs from the Gregorian only in the substitution of a convex hyperboloidal mirror for a concave ellip- soidal mirror as the small speculum. This form has two Casae- distinct advantages: (i) if spherical mirrors are employed Cass their aberrations have a tendency to correct each other; grafts. (2) the instrument is shorter than the Gregorian, caeteris paribus, by twice the focal length of the small mirror. Fewer telescopes have been made of this than perhaps of any other form of reflector; but in comparatively recent years the Cassegrain has acquired importance from the fact of its adoption for the great Melbourne telescope, and from its employment in the 6o-in. reflector of the Mount Wilson Solar Observatory (see below). For spectroscopic purposes the Cassegrain form has peculiar advantages, because in consequence of the less rapid convergence of the rays after reflection from the convex hyperboloidal mirror, the equivalent focus can be made very great in comparison with the length of the tube. This permits the employment of a spectroscope furnished with a collimator of long focus. The magnifying power is computed by the same formula as in the case of the Gregorian telescope. The Newtonian telescope is represented in Fig. 8. A A is a con-cave mirror whose axis is a a. Parallel rays falling on A A converge on the plane mirror B B, and are thence reflected at New right angles to the axis, forming an image in the focus of toalea. the eye-piece E. The surface of the large mirror should be a paraboloid of revolution, that of the small mirror a true optical plane. The magnifying power is = Fie. This form is employed in the construction of most modern reflecting telescopes. A glass prism of total reflection is sometimes substituted for the plane mirror. The Herschelian or front view reflector is represented in fig. 9. A A is a concave parabolic mirror, whose axis a c is inclined to the axis of the tube a b so that the image of an object in the focus of the mirror may be viewed by an eye-piece at E, the angle b a c being Her- equal to the angle c a E. This form was adopted by the Her- n. elder Herschel to avoid the loss of light from reflection in scheila the small mirror of the Newtonian telescope. The front view telescope, however, has hardly been at all employed except by the Herschels. But at the same time none but the Herschels have swept the whole sky for the discovery of faint nebulae; and A probably no other astronomers have worked for so many hours on end for so many nights as they did, and they emphasize the easy position of the observer in using this form of instrument. Construction of Specula. The composition of metallic specula in the present day differs very little from that used by Sir Isaac Newton. Many different alloys have been suggested, some including silver, nickel, zinc or arsenic; but that which has practically been found best is an alloy of four equivalents of copper to one of tin, or the following pro-portions by weight: copper 252, tin 117.8. Such speculum metal is exceedingly hard and brittle, takes a fine white polish, and when protected from damp has little liability to tarnish. The process of casting and annealing, in the case of the specula of the great Melbourne telescope, was admirably described by Dr Robinson in Phil. Trans., 1869, 159, p. 135. Shaping, polishing and figuring of specula are accomplished by methods and tools very similar to those employed in the construction of lenses. The reflecting surface is first ground to a spherical form, the parabolic figure being given in the final process by regulating the size of the pitch squares and the stroke of the polishing machine. Soon after Liebig's discovery of a process for depositing a film of pure metallic silver upon glass from a salt of silver in solution, Steinheil (Gaz. Univ. d'Augsburg, 24th March 1856), and later, in-dependently, Foucault (Comptes Rend us, vol. xliv., February 1857), proposed to employ glass for the specula of telescopes, the reflect-mg surface of the glass speculum to be covered with silver by Liebig's process. Those silver-on-glass specula are now the rivals of the achromatic telescope, and it is not probable that many telescopes with metal specula will be made in the future. The best speculum metal and the greatest care are no guarantee of freedom from tarnish, and, if such a mirror is much exposed, as it must be in the hands of an active observer, frequent repolishing will probably be necessary. This involves refiguring, which is the most delicate and costly process of all. Every time, therefore, that a speculum is repolished, the future quality of the instrument is at stake; its focal length will probably be altered, and thus the value of the constants of the micrometer also have to be redetermined. Partly for these reasons the reflecting telescope with metallic mirror has never been a favourite with the professional astronomer, and has found little employment out of England.' In England, in the hands of the Herschels, Rosse, Lassell and De la Rue it has done ' There is a noteworthy exception in the case of the 18-in. speculum-metal mirror employed by Sir William Huggins at 'Pulse Hill, with which a large part of his remarkable and important series of astrospectroscopic results have been obtained. So far as we know, this mirror has never been repolished since its first installation in 1870, and still retains its admirable surface. One of Short's mirrors, made about 176o or 1770, of 6-in. aperture, now in the possession of Sir William Huggins, has surfaces which still retain their original perfection although they have never been repolished.splendid service, but in all these cases the astronomer and the instrument-maker were one. The silver-on-glass mirror has the enormous advantage that it can be resilvered with little trouble, at small expense, and without danger of changing the figure. Glass is lighter, stiffer, less costly and easier to work than speculum metal. Silvered mirrors have also some advantage in light grasp over those of speculum metal, though, aperture for aperture, the former are inferior to the modern object-glass. Comparisons of light grasp derived from small, fresh, carefully silvered surfaces are sometimes given which lead to illusory results, and from such experiments Foucault claimed superiority for the silvered speculum over the object-glass. But Sir David Gill found from experience and careful comparison that a silvered mirror of 12-in. aperture, mounted as a Newtonian telescope (with a silvered plane for the small mirror), when the surfaces are in fair average condition, is equal in light grasp to a first-rate refractor of to-in. aperture, or area for area as 2: 3. This ratio will become more equal for larger sizes on account of the additional thickness of larger object-glasses and the consequent additional absorption of light in transmission. Mounting of Telescopes. The proper mounting of a telescope is hardly of less importance than its optical perfection. Freedom from tremor, ease and delicacy of movement and facility of directing the instrument to any desired object in the heavens are the primary qualifications. Where accurate differential observations or photographs involving other than instantaneous exposures have to be made, the additional condition is required that the optical axis of the telescope shall accurately and automatically follow the object under observation in spite of the apparent diurnal motion of the heavens, or in some cases even of the apparent motion of the object relative to neighbouring fixed stars. Our limits forbid a historical account of the earlier endeavours to fulfil these ends by means of motions in altitude and azimuth, nor can we do more than refer to mountings such as those employed by the Herschels or those designed by Lord Rosse to over-come the engineering difficulties of mounting his huge telescope of 6 ft. aperture. Both are abundantly illustrated in most popular works on astronomy, and it seems sufficient to refer the reader to the original descriptions.2 We pass, therefore, directly to the equatorial telescope, the instrument par excellence of the modern extra-meridian astronomer. The article TRANSIT CIRCLE describes one form of mounting in which the telescope is simply a refined substitute for the sights or pinules of the old astronomers. The present article contains a description of the mounting of the various forms of the so-called zenith telescope. In its simplest form the mounting of an equatorial telescope consists of an axis parallel to the earth's axis, called "the polar axis"; a second axis at right angles to the polar axis called " the declination axis "; and the telescope tube fixed at right angles to the declination axis. In Fig. so A A is the polar axis; the telescope is attached to the end of the declination axis; the latter rotates in bearings which are attached to the polar axis and concealed by the telescope itself. The telescope is counterpoised by a weight attached to the opposite end of the declination axis. The lower pivot of the polar axis rests in a cup-bearing at C, the upper bearing upon a strong metal casting M M attached toa stone pier S. A vertical plane passing through A A is therefore in the meridian, and the polar axis is inclined to the horizon at an angle equal to that of the latitude of the place of observation. Thus, when the declination axis is horizontal the telescope moves in the plane of the meridian by rotation on the declination axis only. Now, if a graduated circle B B is attached to the declination axis, together with the necessary verniers or microscopes V V for reading it (see TRANSIT CIRCLE), so arranged that when the telescope is turned on the declination axis till its optical axis is parallel to A A the vernier reads 0° and when at right angles to A A 90°, then we can employ the readings of 2 Herschel, Phil. Trans., 1795, 85, p. 347; Rosse, Phil. Trans_ 184o, p. 503; 1861, p. 681. A this circle to measure the polar distance of any star seen in the telescope, and these readings will also be true (apart from the effects of atmospheric refraction) if we rotate the instrument through any angle on the axis A A. Thus one important attribute of an equatorially mounted telescope that, if it is directed to any fixed star, it will follow the diurnal motion of that star from rising to setting by rotation of the polar axis only. If we now attach to the polar axis a graduated circle D D, called the " hour circle," of which the microscope or vernier R reads on when the declination axis is horizontal, we can obviously read off the hour angle from the meridian of any star to which the telescope may be directed at the instant of observation. If the local sidereal time of the observation is known, the right ascension of the star becomes known by adding the observed hour angle to the sidereal time if the star is west of the meridian, or subtracting it if east of the meridian. Since the transit circle is preferable to the equatorial for such observations wherein great accuracy is required, the declination and hour circles .3f an equatorial are employed, not for the determination of the right ascensions and declinations of celestial objects, but for directing the telescope with ease and certainty to any object situated in an approximately known position, and which may or may not be visible to the naked eye, or to define approximately the position of an unknown object. Further, by causing the hour circle, and with it the polar axis, to rotate by clockwork or some equivalent mechanical contrivance, at the same angular velocity as the earth on its axis, but in the opposite direction, the telescope will, apart from the effects of refraction, automatically follow a star from rising to setting. Types of Equatorials.—Equatorial mountings may be divided into six types. (A) The pivots or bearings of the polar axis are placed at its extremities. The declination axis rests on bearings attached to opposite sides of the polar axis. The telescope is attached to one end of the declination axis, and counterpoised by a weight at the other end, as in fig. io. (B) The polar axis is supported as in type A; the telescope is placed between the bearings of the declination axis and is mounted symmetrically with respect to the polar axis ; no counterpoise is therefore requisite. (C) The declination axis is mounted on the prolongation of the upper pivot of the polar axis; the telescope is placed at one end of the declination axis and counter-poised by a weight at the other end. (D) The declination axis is mounted on a forked piece or other similar contrivance attached to a prolongation of the upper pivot of the polar axis; the telescope is mounted between the pivots of the declination axis. (E) The eye-piece of the telescope is placed in the pivot of the polar axis; a portion or the whole of the axis of the telescope tube coincides with the polar axis. (F) The telescope is fixed and the rays are reflected along its axis from an external mirror or mirrors. Mountings of types A and B—that is, with a long polar axis sup-ported at both ends—are often called the " English mounting," and type C, in which the declination axis is placed on the extension of the upper pivot of the polar axis, is called the " German mounting," from the first employment of type C by Fraunhofer. A description of some of the best examples of each type will illustrate their relative advantages or peculiarities. Type A.—Fig. io may be taken as a practical example of the earlier equatorials as made by Troughton in England and afterwards by Gambey for various Continental observatories. In the Phil. Trans. for 1824 (part 3, pp. 1–412) will be found a description by Sir John Herschel and Sir James South of the equatorial telescope which they employed in their measurements of ~.s double stars. The polar axis was similar in shape to that of fig. lo *F' f91Al~llli~SN,, g• and was composed of sheets of tinned iron. In Smyth's celebrated ` ` 71,1 d" f7ilRN lil Bedford telescope the polar axis was ,i'OrSh, fi ;i I of mahogany. Probably the best l innsv example of this type of mounting - ,,,.. , _ T applied to a refractor is that made F1c. i 1.-Melbourne Reflector. by the elder Cooke of York for Flet- cher of Tarnbank; the polar axis is of cast iron and the mounting very satisfactory and convenient, but unfortunately no detailed description has been published. In recent Great years no noteworthy refractors have been mounted on this plan; but type A has been chosen by Grubb for the Mel- great Melbourne reflector, of 48-in. aperture, with marked bourne ingenuity of adaptation to the peculiar requirements telescope. of the case. Fig. i i shows the whole instrument on a small scale with the telescope directed to the pole, and the hour circle set 6^ from the meridian. Type B.—The most important examples of type B are Airy's equatorial at Greenwich (originally made to carry a telescope of aperture, but now fitted with a telescope by Grubb of 28-in. aperture), and the photographic equatorials of 13-in. aperture employed at Paris and other French observatories, of which the object-glasses were made by the brothers Henry and the mountings of any importance to be provided with clockwork. The instrument, by Gautier of Paris. shown in fig. 13, is described in detail by Struve (Beschreibung des These instruments have done admirable work in connexion with auf der Sternwarte zu Dorpat befndlichen grossen Refractors von the great international undertaking, the Carte du Ciel. The general construction will be understood from fig. 12. The double polar axis is composed of hollow metal beams of triangular section. The hour circle has two toothed circles cut upon it, one acted upon by a worm screw mounted on the pier and driven by clockwork, the other by a second worm screw attached to the polar axis, which can be turned by a handle in the observer's hand and thus a slow movement can be given to the telescope in right ascension irides After an illustration in La Nature, by permission of Masson et Cie. pendently of the clock. Slow motion in declination can be communicated by a screw acting on a long arm, which latter can be clamped at pleasure to the polar axis. An oblong metallic box fitted with pivots, whose bearings are attached to the triangular beams, forms the tube for two parallel telescopes; these are separated throughout their length by a metallic diaphragm. The chromatic aberration of the object-glass of one of these telescopes is corrected for photographic rays, and the image formed by it is received on a highly sensitive photographic plate. The other telescope is corrected for visual rays and its image is formed on the plane of the spider-lines of a filar micrometer. The peculiar form of the tube is eminently suited for rigid preservation of the relative parallelism of the axes of the two telescopes, so that, it the image of a certain selected star is retained on the intersection of two wires of the micrometer, by means of the driving clock, aided by small corrections given by the observer in right ascension and declination (required on account of irregularity in the clock movement, error in astronomical adjustment of the polar axis, or changes in the star's apparent place produced by refraction), the image of a star will continue on the same spot of the photographic film during the whole time of exposure. In these telescopes the photographic object-glass has an aperture of 13 in. and the visual object-glass of 10 in. Both telescopes have the same focal length, viz. 11.25 ft., so that, in the image produced, 1 mm. is of arc. An excellent mounting of type B, made by T. Cooke & Sons of York, has been employed by Franklin Adams for making his maps of the sky. Type C.—Many more telescopes have been made of type C than of any other, and this form of mounting is still most generally employed for the mounting of modern refractors. Fraunhofer's chef-d'oeuvre, the great Dorpat refractor, made for Otto Struve about 182o, had a mounting of this type, and was the first equatorial Fraunhofer, Dorpat, 1825), and was an enormous advance upon all previous telescopes for micrometric research. In the hands of Struve results were obtained by it which in combined quality and quantity had never before been reached. Its success was such that the type of Fraunhofer's telescope became stereotyped for many years not only by Fraunhofer's successors but throughout Germany. When, twelve years afterwards, Struve ordered the 15-in. refractor for the new observatory at Pulkovo, the only important change made by Fraunhofer's successors was, at Struve's suggestion, the substitution of a stone pier for the wooden stand in the original instrument. Both the Dorpat and the Pulkovo refractors are defective in rigidity, especially in right ascension. The declination circle is most inconvenient of access, and slow motion in declination can only be effected when the instrument is clamped by a long and inconvenient handle; so that, practically, clamping in declination was not employed. The slow motion in right ascension is defective, being accomplished in the Dorpat refractor by changing the rate of the clock, and in the Pulkovo refractor by a handle which, when used, affects very injuriously the rate of the clock for the time being. Struve's skill as an observer was such that he used to complete the bisection on the fixed wire of the micrometer by a pressure of the finger on the side of the tube—a method of proved efficiency in such hands, but plainly indicative of the want of rigidity in the instrument and of the imperfection of the slow motions (see MICROMETER). The driving circle is also much too small, so that a very slight mechanical freedom of the screw in the teeth involves a large angular freedom of the telescope in right ascension, while its position at the lower end of a too weak polar axis tends to create instability from torsion of that axis. Strange to say, the wooden tube long retained its place in German telescope-mountings. About 184o a great advance was made by the Repsolds of Hamburg in the equatorial mounting of the Oxford heliometer. The driving circle was greatly increased in diameter and placed at the upper end of the polar axis, and both the polar and declination axes were made much stronger in proportion to the mass of the instrument they were designed to carry. (A figure of the instrument is given in the Oxford Observations for 185o.) About 1850 Thomas Cooke of York began his career as a maker of equatorial telescopes. The largest example of his work is the refractor of 24-in. aperture, originally made for the private observatory of Robert Stirling Newall at Gateshead, Northumberland, and afterwards presented by him to the University Observatory, Cambridge. Cooke's mounting is admirable for its symmetry and simplicity of design, its just apportioning of strength, and a general suitability of means to ends. It is not a little curious that the obvious improvement of trans-ferring the declination axis as well as the declination-clamp to the telescope end of the declination axis was so long delayed; we can explain the delay only by the desire to retain the declination circle as a part of the counterpoise. We believe the first important equatorials in which the declination was read from the eye-end were the 15-in. by Grubb and the 6-in. by Cooke, made for the observatory of Lord Crawford (Lord Lindsay) at Dun Echt, Aberdeenshire, about 1873. The plan is now universally adopted. Telescopes of such dimensions can be conveniently directed to any object by the circles without the observer being under the necessity to climb a special ladder. But when much larger instruments are required the hour circle becomes inaccessible from the floor, and means have to be devised for reading both circles from the eye-end. This was first accomplished by Grubb in the great refractor of 27-in. aperture which he constructed for the Vienna observatory, represented in section in fig. 14. The observer's eye is applied to the small telescope E, which (by means of prisms numbered 1, 2, 3, 4) views the vernier attached to the cross-head simultaneously with the hour circle attached to the upper end of the polar axis. Light to illuminate the vernier and circle is thrown from the lamp L upon prism 4 by the prisms 6 and 5. Prism i is in the axis of the declination circle and always reflects rays along that axis, whatever the position of the telescope may be, whilst the prisms 2, 3, 4, 5 and 6 are attached to the cross-head and therefore srd. Y a~au s~ preserve their relative positions to each other. Through the eye-piece of the bent' telescope E' another hour circle attached to the lower end of the polar axis can be seen; thus an assistant is able to direct the telescope by a handle at H to any desired hour angle. A slight rotatory motion of the telescope E on its axis enables the vernier of the declination circle to be read through prism i. The leading features of this fine instrument represent those of all Grubb's large telescopes. The mode of relieving the friction of the declination axis is similar to that employed in the Melbourne telescope and in the account of the Vienna telescope published by Grubb. The end friction of the polar axis is relieved by a ring of conical rollers shown in section beside the principal figure. From this point we must condense farther description into critical remarks on a few typical modern instruments. (I) Telescopes of Moderate Size for Micrometric Research Only.--Fig. 15 shows the mounting of the 8-in. refractor, of 9-ft. focal length, at the private observatory of Dr Engelmann, Leipzig. The object-glass is by Messrs Clark of Cambridge, Mass., the mounting by the Repsolds of Hamburg. The declination circle reads from the eye-end, and four handles for clamping and slow motion in right ascension and declination are situated near the observer's hands. The tube is of sheet steel, light, stiff, and free from tremor. The eye-end carries the micrometer with an illuminating apparatus similar to that described under MICROMETER. The lamp near the eye-end illuminates the field or the wires at pleasure, as well as the position circle of the micrometer and the declination circle; a separate lamp illuminates the hour circle. An excellent feature is the short distance Itween the eye-piece and the declination axis, so that In the bent telescope refracting prisms are employed at the corners to change the direction of the rays. Repsolds' small equatorial. the observer has to follow the eye-end in a comparatively small circle; another good point is the flattening of the cast-iron centre-piece of the tube so that the flange of the declination axis is attached as near to the axis of the telescope tube as is consistent with free passage of the cone of rays from the object-glass. The substitution of small incandescent electric lamps is an improvement now uni- versally adopted. (2) Telescopes for General Purposes.—The modern equatorial should, for general purposes, be capable of carrying spectroscopes of considerable weight, so that the proportional strength of the axes and the rigidity of the instrument have to be considerably increased. The original mounting of the Washington refractor of 26-in. aperture and 322-ft. focal length (described in Washington Observations, 1874, App. 1) was in these respects very defective, the polar and declina- tion axes being only 7 in. in diameter. The great Pulkovo refractor (fig. 16) erected in 1885 is of 3o-in. aperture and 45- ft. focal length. The object-glass is by Clark, the mounting by the Repsolds. The tube is cylindrical, of riveted steel plate, graduated in thickness from the centre to its extremities, and bolted by very powerful flanges to a strong short cast-iron central tube, in which, as in Dr Engelmann's telescope (fig. 15), the attachment to the flange of the declination axis is placed as close as it can be to the axis of the tube without interfering with rays converging from the object-glass to any point in the field of view. A new feature in this instrument is the platform at the lower end of the polar axis, where an assistant can view the hour circle by one eye- FIG. 15.—Dr Engelmann's piece and the declination circle 8-in. Refractor. by another (looking up the per- forated polar axis), and where he can also set the telescope to any hour angle by one wheel, or to any declination by a second, with the greatest ease. The observer at the eye-end can also read off the hour and declination circles and communicate quick or slow motions to the telescope both in right Pulkovo ascension and declination by conveniently refractor. placed handles. The eye end presents an appearance too complicated to be figured here; it has a micrometer and its illumination for the position circle, a micrometer head, and a bright or dark field, clamps in right ascension and declination and quick and slow motion in the same, a finder, micro-scopes for reading the hour and declination circles, an illuminated dial showing sidereal time and driven by an electric current from the sidereal clock, and counter weights which can be re-moved when a spectroscope or other heavy appliance is added. All these, although making up an apparently complicated apparatus, are conveniently arranged, and are all necessary for the quick and easy working of so large an instrument. We have the authority of Otto Struve for stating that in practice they are all that can be desired. There is in this instrument a remarkably elegant method of relieving the friction of the polar axis. Let A A(fig. 17) be a section of the polar axis; it is then easy to adjust the weight P attached to its lower end so that the centre of gravity X of the whole moving parts of the instrument shall be in the vertical (V V) of a line passing through the apex of the hollowed flange p q at q, which flange forms part of the polar axis. If now a wheel W is forced up against q`with a pressure equal to the weight of the moving part of the instrument, the whole weight of the moving part would rest upon W in unstable equilibrium; or if a pressure R, less than W, is employed, we have the end friction on the lower bearing removed to an extent =R sin rp, and the friction on the bearings of the upper pivot removed to the extent of F cos 4,—where is the latitude of the place. The wheel W is therefore mounted on a guided rod, which is forced upwards by suitable levers and weights, and this relief of pressure is precisely proportional to the pressure on the respective bearings. The Repsolds find it unnecessary to relieve the friction of the declination axis. In such large telescopes it becomes a matter of the first importance to provide means of convenient access to the eye-end of the instrument. This the Repsolds have done in the Pulkovo telescope by means of two platforms, as shown in fig. 16. These platforms are capable of easy motion so that the astronomer may be conveniently situated for observing an object at any azimuth or altitude to which the telescope may be directed. For the great refractor more recently erected at Potsdam, Messrs Repsold arranged a large platform mounted on a framework which is moved in azimuth by the dome, so that the observer on the platform is always opposite the dome-opening. This framework is provided with guides on which the platform, whilst preserving its horizontality, is v raised and lowered nearly in an arc of a circle of which the point of intersection of the polar and declination axes is the centre. The rotation of the dome, and p. with it the platform-framework, is accomplished by means of electric motors, as also is the raising and lowering of the platform on its framework. The current is supplied by accumulators, and the switch-board is attached to the platform in a position convenient for use by the astronomer or his assistant. In the original design sup-plied for the 36-in. telescope of the Lick Observatory at Mount Hamilton, California, Grubb suggested that the whole floor, 70 ft. in diameter, should be raised and lowered by water power, under control of the observer by means of electric keys which act on secondary mechanism that in turn works the valves and reversing gear of the water engines. Other water engines, similarly connected, with keys at the observer's hands, rotate the dome and perform the quick motions in right ascension and declination. (An illustration showing these arrangements appeared in The Engineer of July 9, 1886.) Grubb's suggestion of the " rising floor " was adopted, although his original plans for the mounting were not carried out; the construction of the mounting, dome, floor, &c., having been entrusted to Messrs Warner & Swasey of Cleveland, Ohio, U.S.A. It has been contended that it is undesirable to move so great a mass as a floor when a platform alone is required to carry the observer. But a floor, however heavy, suspended by three wire ropes and properly balanced over large, well-mounted pulleys, requires an amount of energy to work it which does not exceed that required to operate a platform of moderate dimensions, and there is a freedom, a safety and a facility of working with a complete floor which no partial platform can give. A floor can be most satisfactorily operated by hydraulic means, a platform cannot be so well worked in this way. The best floor mounting we know of is that designed by O. Chadwick for the Victoria Telescope of the Cape Observatory. An account of it will be found in the History and Description of the Cape Observatory. This floor can be raised at the rate of 1 ft. per second or as slowly as the observer desires—whilst in all the large platforms we have seen (Pots-dam and Paris), the rate of shift is tedious and time-consuming. The largest refracting telescope in active use is the Yerkes telescope, with an object-glass of 4o-in. diameter by Alvan Clark & Son of Cambridge, U.S.A., and with a mounting, dome and rising floor by Warner & Swasey of Cleveland, Ohio, U.S.A. The reader will gather a good general idea of the design from fig. 16. The eye-end is shown on the plate, fig. 25. The chief defect in equatorial mountings of type C is that in general they are not capable of continued observing much past the meridian without reversal. This is an unquestionable draw-back when long exposures near the meridian are required. By the
End of Article: INSTRUMENTS
INSUBRES ("Ivoµ(3pes, "Ivvouf pot)

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