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OBJECTIVE, or OBJECT GLASS
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See also:OBJECTIVE, or See also:OBJECT See also:GLASS , the See also:lens of any See also:optical See also:system which first receives the See also:light from the object viewed; in a See also:compound system the rays subsequently See also:traverse the See also:eye-piece. The theoretical investigations upon which the construction of an optical system having specified properties is based, are treated in the See also:article See also:ABERRATION, and, from another standpoint, in the article DIFFRACTION. Here we See also:deal with the methods by which the theoretical deductions are employed by the See also:practical optician. It should be noted that the mathematical calculations provide data which are really only approximations, and consequently it is often found that a system constructed on such data requires modification before it fulfils the practical requirements. For example, take the See also:case of a photographic objective. Calculations of the paths of two extreme rays in the meridional See also:section of an oblique See also:pencil of large See also:aperture may prove that the rays intersect on a See also:plane containing the axial See also:focus, but similar calculations of many other rays would be necessary before the mean point of intersection could be settled with sufficient exactness. Suppose, however, that the optician has accurately realized the results of the mathematician, he can then determine the divergence of the practical from the theoretical properties by measuring the positions and conformation of the most distinct or mean foci, and, if sufficiently acquainted with the theory of the construction, he can modify one or more curvatures or thicknesses and so attain to a closer agreement with the ideal. Theory and practice co-operate in the realization of an See also:original system. The See also:order is See also:net always the same, but generally the mathematician, by notoriously laborious calculations, supplies data which are at first closely followed by the constructor and after-wards modified in accordance with experimental observations. In addition to the problem of constructing an original system, the optician has to deal with the See also:reproduction of a realized system in different sizes. Two questions then arise: (i) To what degree of accuracy the radii of curvature can, or should, be repeated, and (2) to what degree of uniformity the surfaces can, or should be figured. With regard to the first point there is no See also:great difficulty in working the requisite See also:iron or See also:brass toolsin the curvature or the figuring. A rough See also:idea of the exactitude to which the figuring of the finest See also:telescope objectives must be carried out is readily deduced. If two slips of See also:paper, bearing printed letters -fig of an in. high be placed in almost exact See also:alignment, one 31.2 in. from the eye and the other 39 in., and viewed in moderate daylight with the eye having a pupillary aperture of of an in., one set of the letters will be legible while the other is not. In this case the difference of convergence or refracting See also:power exercised by the eye in transferring its focus from one slip to the other is ris or one See also:quarter diopter. If an See also:image on the retina is s diopter out of focus, then each point of the object is represented by a circle of confusion 0.000.4 in. or 2' 45" in angular measure in See also:diameter, the See also:focal length of the eye being assumed to be o•5 in. and the pupillary aperture k of an in. If the effective aperture of the See also:pupil or the aperture of a pencil traversing the pupil be 1/nth of this See also:standard, the See also:size of the disk of confusion will be the same (viz. 0.0004 in.) if the retinal image be n quarter diopters out of focus. In See also:general, for a See also:constant size of the circle of confusion or, in other words, the same amount of visual blurring, the apertures of the pencils traversing the pupil and the focussing errors (expressed in quarter diopters) vary inversely. If a portion of a figured See also:surface of a telescope objective differs in curvature from the See also:major portion of the lens so as to See also:form a circle of confusion on the retina of a diameter not less than 2' 45", it is clear that the lens is faulty, the image formed by the perfect portion being See also:sharp and well defined, and that formed by the imperfect portion blurred to the extent above determined, and to a greater extent if we allow for the effect of diffraction in the formation of the image. For example, a protuberance i in. in diameter at the centre of an object glass of 12 in. aperture refracting to a See also:separate focus would theoretically form a See also:spurious disk of about 5 seconds diameter, which would subtend a diameter of 50 minutes at the retina under a power of 600. Regarding 2' 45" as the maximum diameter of a geometric circle of confusion permissible in a telescopic object glass, we proceed to determine the heights of the protuberance or depression which causes it. If f be the See also:equivalent focal length of the eye-piece and F that of the objective (the back focal length in the case of the See also:microscope), then the linear See also:error at the focus of the eye-piece is 3 igf 2, or, expressed as a variation of 1/F, T g(f/F)2, (=A). If a lens has one See also:side plane and is worked to a mathematically sharp edge, its thickness t at the centre is (approximately) See also:A2/8r, where A is the whole aperture and r the See also:radius; and if g be the equivalent focal length and u the refractive See also:index, we may write r = g (i — I) and obtain t=A2/8g(µ—i) It is clear that for lenses in which the focal length is large compared with the aperture, the thickness t is See also:independent of the shape of the lens so See also:long as the focal length and aperture remain constant. Consequently a protuberance may be regarded as a thin meniscus lens with mathematically sharp edges accurately fitted to a perfectly See also:regular spherical surface. Substituting for 1/g the TiA (f/F)2 obtained above it follows that A2 11)2 t=8(µ_j) 156 \F(z). The effective aperture of the eye has been supposed to be s in.; calling this P, it is then obvious that (since F/f is the magnifying power) P(F/f) is the theoretical aperture of objective requisite to See also:supply the , in. eye-pencil. Substituting P(F/f) for A in See also:equation (2) we obtain t=P2/8(µ- I) X156 (3). This relation gives the thickness of a meniscus protuberance fitted to an objective (assumed to have an unlimited aperture) which fills the I in. pupil and occasions the maximum blurring permissible. Ifµ be 1• t is equal to 1/39,936 in. If the thickness t correspond to the aperture A, then for another aperture a to produce the same blurring we must have 0' (1/F) = A(1/F)A/a, i.e. the focal length of the protuberance, and therefore the thickness t must vary as A. Consider a telescope of 12 in. aperture, focal length of objective (F) =18o in., focal length of eye-piece (f) 0.3 in. and magnifying power (F/f) =600. The aperture theoretically requisite to transmit the pupillary pencil of j in. aperture is •600=75 in. If the permissible protuberance See also:cover the entire aperture of 75 in. its thickness would be 1/39,936 in. as above, but if restricted to a diameter of 1 in., then the maximum allowable thickness would be 1/75X1/39,936 in.=say 1/3,000,000 in. Since the latter protuberance is assumed to fill only , of the aperture of the pupil of the eye, it produces an error in focussing equivalent to 75 quarter diopters or 17h. If we take the power of the eye-piece to be I(•3 in. and subtract from it 75/156, we obtain' 1.35, so that OF is - .05 in. Either the See also:knife-edge test, or the more usual method of testing figuring by examining the out-of-focus disks formed on the retina when the eye-piece is inside and outside its correct focus, would certainly show the effect of this protuberance as a See also:bright central spot when inside focus, and a dark central patch when outside; a practised eye can detect one-See also:half the above error, and a quarter when the power is 1200 instead of boo. It may be noticed that, under the same circumstances, the error permissible in a reflecting telescope is only one quarter of that admitted in the refractor. In the case of a microscope objective of to in. back-focal-length used with a 1 in. eye-piece, the aperture required to transmit the pupillary pencil of a in. aperture is It in. Regarding the supposititious protuberance or depression as -A in. in diameter, its thickness or See also:depth must not exceed 1/39,936 X0.05/1.25, or say I/1,000,000 in. Therefore the accuracy of figuring required in the best micro-scopes does not fall far See also:short of that required in telescopes. The best optical workmanship, as applied to large reflecting surfaces, aims at reducing See also:local protuberances or depressions to within the limiting height or depth of one twelve-millionth See also:part of their diameter (A) and the optical methods which detect these errors are exceedingly delicate. The finest See also:spherometer detects errors down to about three-millionths of an See also:inch, below which it is valueless. The same applies to the study of the interference fringes formed when a See also:master See also:curve is fitted. It will not show up such See also:fine errors. The figuring of spherical surfaces 12 in. or more in diameter by See also:abrasion with a polisher so that no part of the surface is elevated or depressed above the See also:average level by more than the above defined amounts is commonly practised, but much technical knowledge is necessary for success. It is a sine qua non that the material of the polisher should be as plastic and inelastic as is consistent with a moderate degree of hardness. The best material for large See also:work is See also:Stockholm See also:pitch from which the greater part of the See also:turpentine has been removed by evaporation, and the abrasive used is the finest See also:rouge and See also:water. For small work certain waxes, more or less mixed with rouge or See also:putty See also:powder, are used. Water is used as the lubricant. During delicate figuring temperature_ changes must be carefully avoided, otherwise buckling and consequent See also:bad figuring of the lens or a variation in the hardness of the polisher may supervene. The See also:motion of the polisher must therefore be leisurely. Moreover, any surface must be allowed to attain a See also:uniform temperature before testing. When, as often happens, an See also:elevation or depression on a large lens apparently refuses to be dislodged by straightforward polishing, recourse is had to local retouching. The faulty parts are localized by optical tests and then rubbed down by small polishers of an inch or more in diameter. In this way a central protuberance I in. in diameter and 1/2,000,000 of an in. high See also:standing on the centre of a large objective may be removed by a polisher less than an inch in diameter worked at 200 half inch strokes per See also:minute and at a pressure of 6 ozs. in about a minute. Great care is required, for if the See also:process be carried too far, the whole surface must be re-figured. Local retouching serves to remove those conspicuous zones of aberration to which certain photographic lenses of large relative aperture are necessarily liable. An See also:annular channel is polished out at a mean distance equal to of the semi-aperture from the centre of the lens, and this iscarefully shaded off towards the centre and also towards the edge; this corrects the See also:zone of rays which focus at a point short of the focus of the centre and edge rays. This correction is particularly necessary in the case of certain lenses designed for stellar See also:photography. (H. D. Additional information and CommentsThere are no comments yet for this article.
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