SPHEEICAL ABEEEATION. 11 



central part, which, if sufficiently small in proportion 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-achromatic) 

 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 inten- 

 sity 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 issuing from any point of an object that can enter the 

 lens ; on the extent of which angle depend some of the most 

 important qualities of a Microscope (§ 131). — The Spherical Aber- 

 ration may be got rid of altogether, however, by making use of 

 combinations of Jenses, so disposed that their opposite 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 favourable position may be corrected by 

 that of a concave lens of much less power in its most unfavourable 

 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 by a method of this kind that the 

 Optician aims to correct the Spherical Aberration, in the construc- 

 tion 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 it sometimes happens 

 that this correction is not perfectly made ; and the want of it 

 becomes evident in the fog by which the distinctness of the image, 

 and especially the sharpness of its outlines, is impaired. 



11. But the spherical aberration is not the only imperfection 

 with which the Optician has to contend in the construction of 

 Microscopes. A difficulty equally serious arises from the unequal 

 refrangibility of the several Coloured rays which together make 

 up "White or colourless light, * so that they are not all brought to 

 the same focus, even by a lens free from spherical aberration. It 

 is this difference in their refrangibility, which causes their complete 

 separation or ' dispersion ' by the Prism into a Spectrum ; and it 



* It has been deemed better to adhere to the ordinary phraseology, 

 when speaking of this fact, as more generally intelligible than the 

 language in which it might be more scientifically described, and at the 

 same time leading to no practical error. 



