70 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



FIG. 5. 



6 * 



smaller than itself. Further, it is to be remarked, that the 

 larger the image in proportion to the object, the less bright it 



\vill be, because the same 

 amount of light has to be 

 spread over a greater sur- 

 face ; whilst an image that 

 is smaller than the object, 

 will be more brilliant in 

 the same proportion. 



9. The knowledge of 

 these general facts will en- 

 able us readily to under- 



Formation of Images by convex lenses. ^^ ^ ordinary opem _ 



tion of the Microscope; but the instrument is subject to cer- 

 tain optical imperfections, the mode of remedying which can- 

 not be comprehended without an acquaintance with their 

 nature. One of these imperfections results from the une- 

 qual refraction of the rays which have passed through lenses, 

 whose curvatures are equal over their whole surfaces. If 

 the course of the rays passing through an ordinary convex 



6), it will be found that 

 they do not all meet exactly 

 in the foci already stated, 

 but that the focus F of the 

 rays A B, A B, which have 

 passed through the periphe- 

 ral portion of the lens, is 

 much closer to it than that 

 of the rays ab, ab, which are 

 nearer the line of its axis ; 

 so that, if a screen be held 



lens be carefully laid down (] 



FIG. 6. 



Diagram illustrating Spherical Aberration. 



in the former, the rays which have passed through the central 

 portion of the lens will be stopped by it before they have come 

 to a focus ; and if the screen be carried back into the focus of the 

 latter, the rays which were most distant from the axis will have 

 previously met and crossed, so that they will come to it in a state 

 of divergence, and will pass to c and d. In either case, there- 

 fore, the image will have a certain degree of indistinctness ; and 

 there is no one point to which all the rays can be brought by a 

 single lens of spherical curvature. The difference between the 

 focal points of the central and of the peripheral rays, is termed 

 the Spherical Aberration. It is obvious that, to produce the de- 

 sired effect, the curvature requires to be increased around the 

 centre of the lens, so as to bring the rays which pass through it 

 more speedily to a focus ; and to be diminished towards the cir- 

 cumference, so as to throw the focus of the rays influenced by it 

 to a greater distance. The requisite conditions may be theoreti- 

 cally fulfilled by a lens, one of whose surfaces, instead of being 

 spherical, should be a portion of an ellipsoid or hyperboloid of 



