See also:

• SPHEROMETER (Gr. ocPaipa , a sphere, p rpov, a measure)

SPHEROMETER (Gr. ocPaipa , a See also:

• SPHERE (Gr. vclsa?pa, a ball or globe)

sphere, p rpov, a measure)  , an See also:

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

instrument for the precise measurement of the See also:

• RADIUS

radius of a See also:

• SPHERE (Gr. vclsa?pa, a ball or globe)

sphere or the thickness of a thin See also:

• PLATE

plate . The usual See also:

• FORM (Lat. forma)

form consists of a See also:

• FINE

fine 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 moving in a 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 carried on the centre of a small three-legged table; the feet forming the vertices of an equilateral triangle (see figure) . The See also:

• LOWER

lower end of the screw and those of the table legs are finely tapered and terminate in hemispheres, so that each rests on a point . If the screw has two turns of the See also:

• THREAD (0. Eng. praed, literally, that which is twisted, prawan, to twist, to throw, cf. " throwster," a silk-winder, Ger. drehen, to twist, turn, Du. draad, Ger. Draht, thread, wire)

thread to the millimetre the 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 is usually divided into 50o equal parts, so that See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences of o•oot millimetre may be measured without using a See also:

• VERNIER, PIERRE (c. 1580–1637)

vernier . A 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, however, may be fitted, 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 magnify the. See also:

• SCALE
• SCALE (1) A

scale divisions . A See also:

• VERTICAL (from Lat. vertex, highest point)

vertical scale fastened to the table indicates the number of whole turns of the screw and serves as an See also:

• INDEX

index for See also:

• READING

reading the divisions on the head . In order to measure the thickness of a plate the instrument is placed on a perfectly level See also:

• PLANE

plane See also:

• SURFACE

surface and the screw turned until the point just touches; the' exact instant when it does so is defined by a sudden diminution of resistance succeeded by a considerable in-crease . The divided head and scale are read ; the screw is raised ; the thin plate slipped under it; and the See also:

• PROCESS

process is repeated . The difference between the two readings gives the required thickness . A contact- See also:

• LEVER (through O. Fr. leveour, levere, mod. levier, from Lat. levare, to lift, raise)
• LEVER, CHARLES JAMES (1806-1872)

lever, delicate level or electric contact arrangement may be attached to the See also:

• SPHEROMETER (Gr. ocPaipa , a sphere, p rpov, a measure)

spherometer in order to indicate the moment of touching more precisely than is possible by the sense of See also:

• TOUCH (derived through Fr. toucher from a common Teutonic and Indo-Germanic root, cf. " tug," " tuck," O. H. Ger. zucchen, to twitch or draw)

touch . To measure the radius of a sphere—e.g. the curvature of a lens-the spherometer is levelled and read, then placed on the sphere, adjusted until the four points exert equal pressure, and read again . The difference gives the thickness of that portion of the sphere cut off by a plane passing through the three feet .

Calling this distance h, and the distance between the feet a, the radius R is given by the See also:

• FORMULA
• FORMULA (Lat. diminutive of forma, shape, pattern, &c., especially used of rules of judicial procedure)

formula R = (See also:

• A2(z)

a2+3h2)/6h .