CONSTRUCTION OF THE MICROSCOPE. 



27 



coloured rays which together make up white light, so that 

 they are not all brought to the same focus, even by a lens 

 free from spherical aberration. It is, indeed, this differ- 

 ence in their refrangibility which causes their complete 

 separation by the prism into a spectrum. 



The correction of chromatic with spherical aberration is 

 effected in a most ingenious manner, by combining a con- 

 vex 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 common lens, 

 we see that it is bordered with the colours of the rainbow ; 

 or if we look through a common magnifying-glass at the 

 letters on the title-page of a book, we see them slightly 

 coloured at their edges in the same manner. The cause 

 of this iridescent border is that the primitive rays — red, 

 yellow, and blue,— of which a colourless ray of light is com- 

 posed, are not all equally refrangible. Hence they are 

 not all brought to one point or focus, but the blue rays 

 Deing the most refrangible, come to a focus nearer the lens 

 than the yellow ones, which are less refrangible, and the 

 yellow rays than the red, which are the least refrangible. 

 Thus, in fig. 16, chromatic aberration proves still more 



Fig. 16. 



detrimental to the distinct definition of images formed by 

 a lens, than spherical aberration. This arises more from 

 the size of the circles of dissipation, than from the iri- 

 descent border, and it may still exist, although the spherical 

 aberration of the lens be altogether corrected. Chromatic 

 aberration is, as before stated, corrected by combining, in 

 the construction of lenses, two media of opposite form, and 

 differing from each other in the proportion in which they 



