48 THE MICROSCOPE. 



feet definition; and it should be observed that only 

 objectives of the finest construction will bear the 

 deeper eye-pieces. Opticians furnish with most of 

 their instruments two, three, or more oculars : A, B, 

 and C ; these, together with a Kellner or orthoscopic 

 eye-piece, D, the field-lens of which is bi-convex, and 

 therefore gives a larger field, are all the microscopist 

 will require. The Huyghenian eye-piece, which is 

 still in use, consists of two plano-convex lenses, with 

 their plane sides turned towards the eye, and at a 

 distance apart equal to half the sum of their focal 

 lengths, and having a stop or diaphragm midway 

 between the lenses. Huyghens was not aware of the 

 value of his eye-piece ; it was reserved for Boscovicli 



D c A a 



IG. 30. Eye-pieces. 



to point out that, by this important arrangement, he 

 had accidentally corrected a portion of the chro- 

 matic aberration incidental to the earlier forms. Let 

 fig. 31 represent the Huyghenian eye-piece of a micro- 

 scope, f f being the field glass, and e e the eye-glass, 

 and I m n the two extreme rays of each of the three 

 pencils emanating from the centre and ends of the 

 object, of which, but for the field-glass, a series of 

 coloured images would be formed from r r to b b ; 

 those near r r being red, those near b b blue, and the 

 intermediate ones green, yellow, and so on, correspond- 

 ing with the colours of the prismatic spectrum. This 

 order of colours is the reverse of that of the common 

 compound microscope, in which the single object-glass 

 projects the red image beyond the blue. 



