" The mechanism for applying these prin ciples to the correction of an object-glass under the various circumstances is repre sented in fig. 18, where the anterior lens is set in the end of a tube A A, which slides on the cylinder B containing the remainder of the combination ; the tube A A, bolding the lens nearest the object, may then be moved upon L the cylinder for the purpose of varying the distance according to the thickness of the glass covering the object, by turning the screwed ring c a, or more simply by sliding the one on the other, and clamping them together when adjusted. An aperture is made in the 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 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 cliromatism 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." 1.1 In the annexed engraving, fig. 19, we have shown the triple achromatic objective in connection with the eye-piece consisting of the field-glass F 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 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 fig. 20 represent the Huyghenean eye-piece of a microscope; F and E E being the field-glass and eye-glass, and a as 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 n R to it n ; those near R R 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 (fig. 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 it R are curved in the wrong direction to bo 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 n' n' and it' a', 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 E. The field-glass has at the 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 the blue foci a a as much beyond the reel foci it n as the sum of the distancea beta ern the red and blue foci of the field-lens and eye lens; so that the separation n it is exactly taken up in passing through those two laves, and the whole of the colours coincide as to focal distance as soon as the rays have pARIlld the e‘e-lens. But while they coincide as to distance, they differ in another rexpect ; the blue images are ren dered smaller than the red by the superior refractive power of the field-glass upon the blue rays. In tracing the pencil t, for instance, it will be noticed that after pansing the field-glass two nets of lines are drawn. one whole and one dotted, the former representing the reel, and the latter the blue rays. This in the accidental effect in the lluyglienean eye-piece pointed out by lloecovich. This separation into colours at the field-glass is like the over-correction of the objective ; it leads to a subsequent complete correction. For if the coloured rays were kept together till they reached the eye-glass, they would then become coloured, and present coloured images to the eye; but fortu nately. and most beautifully, the separation effected by the field-glass causes the blue rays to fall so much nearer the centre of the where, owing to the spherical figure, the refractive power is less than nt the margin, that the spherical error of the eye-lens constitutes a nearly perfect balance to the chromatic dispersion of the field-lens, and the red and blue rays L' and t.' emerge sensibly parallel, presenting, iu consequence, the perfect definition of a single point to the eye. The Male reasoning is true of the intermediate colours and of the other pencils.