168 



PRACTICAL TKEATISE ON THE MICROSCOPE. 



ciety of Arts. The eye-piece in question was invented by 

 Huyghens for telescopes, with no other view than that of 

 diminishing the spherical aberration by producing the refrac- 

 tions at two glasses instead of one, and of increasing the field 

 of view. It consists of two planoconvex lenses, with their 

 plane sides towards the eye, and placed at a distance apart 

 equal to half the sum of their focal lengths, with a stop or 

 diaphragm placed midway between the lenses. Huyghens 

 was not aware of the value of his eye-piece ; it was reserved 

 for Boscovich to point out that he had, by this important 

 arrangement, accidentally corrected a great part of the chro- 

 matic aberration. Let fig. 118 represent the Huyghenian 



eye-piece of a microscope, 

 F F being the field-glass, 

 and E E the eye-glass, and 

 L M N the two extreme 

 rays of each of the three 

 pencils, emanating firom the 

 centre and ends of the ob- 

 ject, of which, but for the 

 field-glass, a series of co- 

 loured images would be 

 formed from B, R to B B ; 

 those near B. R being red, 

 those near B B blue, and 

 the intermediate ones green, 

 yeUow, and so on, corre- 

 sponding with the colours of 

 the prismatic spectrum. — 

 This order of colours, it will 

 be observed, is the reverse 

 j-j J 18 of that occurring in the com- 



mon compound microscope, 

 represented by fig. 115, in which the single object-glass pro- 

 jects the red image beyond the blue. The effect just 

 described, of projecting the blue image beyond the red, is 

 purposely produced, for reasons presently to be given, and is 

 called over-correcting the object-glass as to colour. It is to 



