637 



MICROSCOPE. 



MICROSCOPE. 



633 



thinnest possible film of talc, as, from the character of the chromatic 

 aberration) it will be seen that varying the distances of the combina- 



Fig. 18. 



M 



tions Vrill not sensibly affect the correction ; though object-lenses may 

 be made to include a given fluid or solid 

 Fig. 19. medium in their correction for colour. 



" The mechanism for applying these prin- 

 ciples to the correction of an object-glass 

 under the various circumstances is repre- 

 sented in fij. 18, where the anterior lens is set 

 E in the end of a tube A A, which slides on the 



cylinder B containing the remainder of the 

 combination ; the tube A A, holding the lens 

 nearest the object, may then be moved upon 

 i*)A the cylinder B, for the purpose of varying the 

 I distance according to the thickness of the 

 I glass covering the object, by turning the 

 J screwed ring c c, or more simply by sliding 

 the one on the other, and clamping them 

 Bi " _. together when adjusted. An aperture is 

 made in th tube A, within which is seen a 

 mark engraved on the cylinder, and on the 

 edge of which are two marks, a longer and a 

 shorter, engraved upon the tube. When the 

 mark on the cylinder coincides with the 

 longer mark on the tube, the adjustment is 

 perfect for an uncovered object ; and when 

 the coincidence is with the short mark, the 

 proper distance is obtained to balance the 

 aberrations produced by glass ^-.th of an inch 

 thick, and such glass can be readily supplied. 

 " It is hardly necessary to observe, that the 

 necessity for this correction is wholly inde- 

 pendent of any particular construction of 

 the object-glass ; as in all cases where the 

 object-glass is corrected for an object un- 

 covered, any covering of glass will create a 

 different value of aberration to the first 

 lens, which previously balanced the aberration 

 resulting from the rest of the lenses ; and as 

 this disturbance is effected at the first re 

 fraction, it is independent of the other part 

 of the combination. The visibility of the 

 effect depends on the distance of the object 

 from the object-glass, the angle of the pencil 

 transmitted, the focal length of the com- 

 bination, the thickness of the glass covering 

 the object, and the general perfection of the 

 corrections for chromatisiu and the oblique 

 ' pencils. 



" With this adjusting object-glass, there- 

 fore, we can have the requisites of the 

 greatest possible distance between the object 

 and object-glass, an intense and sharply 

 defined image throughout the field from the 

 large pencil transmitted, and the accurate 

 correction of the aberrations; also, by the 

 adjustment, the means of preserving that 

 correction under all the varied circumstances 

 in which it may be necessary to place an 

 object for the purpose of observation." 

 ,, In the annexed engraving, Jig. 19, we have 



shown the triple achromatic objective in 

 connection with the eye-piece consisting of the field-glass r F and the 



eye-glass E E, forming together the modern achromatic telescope. 

 The course of the light is shown by drawing three rays from the 

 centre and three from each end of the object o. These rays would, 

 if left to themselves, form an image of the object at A A, but being 

 bent and converged by the field-glass F P, they form the image at B B, 

 where a stop is placed to intercept all light except what is required 

 for the formation of the image. From B B, therefore, the rays proceed 

 to the eye glass exactly as has been described in reference to the simple 

 microscope and to the compound of two glasses. 



If we stopped here we should convey a very imperfect idea of the 

 beautiful series of corrections effected by the eye-piece, and which were 

 first pointed out in detail in a paper on the subject published by Mr. 

 Varley in the 51st volume of the ' Transactions of the Society of Arts.' 

 The eye-piece in question was invented by Huyghens for telescopes, 

 with no other view than that of diminishing the spherical aberration 

 by producing the refractions at two glasses instead of one, and of 

 increasing the field of view. It was reserved for Boscovich to point 

 out that Huyghens had by this arrangement accidentally corrected a 

 great part of the chromatic aberration; and this subject is further 

 investigated with much skill in two papers by Professor Airy in the 

 ' Cambridge Philosophical Transactions," to which we refer the mathe- 

 matical reader. These investigations apply chiefly to the telescope, 

 where the small pencils of light and great distance of the object 

 exclude considerations which become important in the microscope, 

 and which are well pointed out in Mr. Varley's paper before 

 mentioned. 



Let fly. 20 represent the Huyghenean eye-piece of a microscope ; F p 



and E E being the field-glass and eye-glass, and L n N the two extreme 

 rays of each of the three pencils, emanating from the centre and ends 

 of the object, of which, but for the field-glass, a series of coloured 

 images would be formed from R R to B B ; those near R it being red, 

 those near B B blue, and the intermediate ones green, yellow, and so 

 on, corresponding with the colours of the prismatic spectrum. This 

 order of colours, it will be observed, is the reverse of that described in 

 treating of the common compound microscope (jig. 12), in which the 

 single object-glass projected the red image beyond the blue. The 

 effect just described, of projecting the blue image beyond the red, is 

 purposely produced for reasons presently to be given, and is called 

 over-correcting the object-glass as to colour. It is to be observed, also, 

 that the images B B and R R are curved in the wrong direction to be 

 distinctly seen by a convex eye-lens, and this is a further defect of the 

 compound microscope of two lenses. But the field-glass, at the same 

 time that it bends the rays and converges them to foci at B' B' and B' R', 

 also reverses the curvature of the images as there shown, and gives 

 them the form best adapted for distinct vision by the eye-glass E B. 

 The field-glass has at tho same time brought the blue and red images 

 closer together, so that they are adapted to pass uncoloured through 

 the eye-glass. To render this important point more intelligible, let it 

 be supposed that the objective had not been over-corrected, that it had 

 been perfectly achromatic ; the rays would then have become coloured 

 as soon as they had passed the field-glass ; the blue rays, to take the 

 central pencil for example, would converge at b and the red rays at r, 

 which is just the reverse of what the eye-lens requires ; for as its blue 

 focus is also shorter than its red, it would demand rather that the 

 blue image should be at r and the red at b. This effect we have shown 

 to be produced by the over-correction of the objective, which protrudes 



