32 THE MICROSCOPE. 



system theoretical and practical difficulties have been 

 overcome in the construction of the modern microscope, 

 and which, until quite lately, were thought to be insur- 

 mountable, thus greatly adding to the value of the 

 instrument as a means of scientific research. 



Chromatic Aberration. A far greater difficulty arises 

 from the unequal ref rangibility of the different coloured 

 rays which together make up white light. It is this 

 difference in refrangibility that produces a complete 

 separation of rays by the prism, forming the spectrum. 



The correction of chromatic and spherical aberration 

 is effected in a very ingenious manner, by combining a 

 convex lens made of crown-glass, and a concave lens of 

 flint-glass. If we examine closely the image projected 

 on the table of a camera obscura provided with a com- 

 mon lens, we see that it is fringed with the colours of 

 the rainbow ; again, if we look through a common mag- 



FJG. 18. 



nifying-glass at the letters on the title-page of a book, 

 we see them slightly coloured at their edges in a similar 

 manner. The cause of this iridescent border is that 

 the primitive rays red, yellow, and blue, of which a 

 colourless ray of light is composed, are not equally re- 

 frangible. Hence they are not simultaneously brought to 

 one point or focus; the blue rays being the most refran- 

 gible, come to a focus nearer the lens than the yellow, 

 which are less refrangible, and the yellow rays than 

 the red, which are the least refrangible. It is seen, 

 in fig. 18, chromatic aberration proves still more detri- 

 mental to the distinct definition of images formed by 

 a lens, than spherical aberration. This arises more 



