8 THE MICROSCOPE AND ITS REVELATIONS. 



the aperture or working-surface of the lens, so as to employ only the rays 

 that pass through its central part, which, if sufficiently small in propor- 

 tion to the whole sphere, will bring them all to nearly the same focus. 

 Such a reduction is made in the Object-glasses of common (non-achro- 

 matic) Microscopes; in which, whatever be the size of the lens itself, the 

 greater portion of its surface is rendered inoperative by a stop, which is a 

 plate with a circular aperture interposed between the lens and the rest of 

 the instrument. If this aperture be gradually enlarged, it will be seen 

 that, although the image becomes more and more illuminated, it is at the 

 same time becoming more and more indistinct; and that, in order to gain 

 defining power, the aperture must be reduced again. Now, this reduction 

 is attended with two great inconveniences: in the first place, the loss of 

 intensity of light, the degree of which will depend upon the quantity 

 transmitted by the lens, and will vary therefore with its aperture; and, 

 secondly, the diminution of the Angle of Aperture, that is, of the angle 

 a b c (Fig. 10) made by the most diverging of the rays of the pencil issu- 

 ing from any point of an object, that can enter the lens and take part in 

 the formation of an image of it; on the extent of which angle (as will be 

 shown hereafter) depend some of the most important qualities of a Micro- 

 scope. " 



11. The Spherical Aberration may'be approximately corrected, how- 

 ever, by making use of combinations of lenses, so disposed that their op- 

 posite aberrations shall correct each other, whilst magnifying power is 

 still gained. For it is easily seen that, as the aberration'of a concave 

 lens is just the opposite of that of a convex lens, the aberration of a con- 

 vex lens placed in its most favorable position may be corrected by that of 

 a concave lens of much less power in its most unfavorable position; so 

 that, although the power of the convex lens is weakened, all the rays 

 which pass through this combination will be brought to one focus. It is 

 thus that the Optician aims to correct the Spherical Aberration, in the 

 construction of those combinations of lenses which are now employed as 

 Object-glasses in all Compound Microscopes that are of any real value as 

 instruments of observation. But this correction is not always perfectly 

 made: and the want of it becomes evident in ihefog by which the dis- 

 tinctness of the image, and especially the sharpness of its outlines, is im- 

 paired; and in the eidola, or false images, on each side of the best focal 

 point, which impair the perfection of the principal image, and can be 

 themselves brought into view when proper means are used for their de- 

 tection. 1 The skill of the best constructors of Microscopic objectives has 

 been of late years successfully exerted in the removal of the * residual 

 errors' to which these eidola were due; so that objectives of the largest 

 angular aperture are now made truly aplanatic, the corrections for Sphe- 

 rical Aberration being applied with/a perfection which was formerly sup- 

 posed to be attainable only in the case of Objectives of small or moderate 

 aperture. Still, the difficulty (and the consequent cost) of producing 

 such objectives, constitutes one out of many reasons for the preference 

 of objectives ef moderate aperture, in which the correction for Spherical 

 Aberration can be easily made complete, for all the ordinary purposes of 

 scientific investigation ( 17). 



12. But spherical aberration is not the only difficulty with which the 

 Optician has to contend in the construction of Microscopes; for one 



1 See Dr. Royston Pigott's description of his " Searcher for Aplanatic Images,"" 

 and its uses, in the "Philos. Transact." for 1870, p. 59. 



