OPTICAL PRINCIPLES OP THE MICROSCOPE. 



favorable position may be corrected by a concave lens of much 

 less power in its most unfavoi-able position ; so that, although 

 the povs^er 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 construction of those combinations 

 of lenses which are now employed as object-glasses, in all Com- 

 pound 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 imaga of the object, and 

 especially the precision of its outlines, is obscured. 



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 

 refrangihility of the several colored rays, which together make 

 up white or colorless light,' so that they are not all brought to 

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

 It is this diflerence in their refrangihility, which causes their 

 complete separation 1)}' the prism into a spectrum ; and it mani- 

 fests itself, though in a less degree, in the image formed by a 

 convex lens. For if parallel rays of white light fall upon a con- 

 vex surface, the most refrangible of its component rays, namely, 

 the violet, will be brought to a focus at a point somewhat nearer 

 to the lens than the principal focus, which is the mean of the 

 whole ; and the converse will be true of the red rays, which are 

 the least refrangible, and whose focus will therefore be more dis- 

 tant. Thus in Fig. 7, the rays of white light, a b, a b, which fall 



on the peripheral portion 

 of the lens, are so far de- 

 composed, that the violet 

 rays are brought tea focus 

 at c, and crossing there, 

 diverge again and pass on 

 towards f f. On the other 

 hand, the red rays are not 

 brought to a focus until D, 



Diagram illustriiting Chromatic Aberration, and "crOSS the diverging 



violet rays at e e. The foci of the intermediate rays of the spec- 

 trum (indigo, blue, green, yellow, and orange) are intermediate 

 between these two extremes. If the image he received upon a 

 screen placed at c, the focus of the violet rays, violet will pre- 

 dominate in its own color, and it will be surrounded by a pris- 

 matic fringe in which blue, green, yellow, orange, and red may 

 be successively distinguished. If, on the other hand, the screen 

 be placed at d, the focus of the red rays, the image will have a 



' It has been deemed better to adhere to the ordinary phraseology, when speaking of 

 this fact, as more generally iiitelMgible than the language in whicli it might be more 

 scientifically described, and at the same time leading to no practical error. 



Fig. 7. 



