CONSTRUCTION OF THE MICROSCOPE. 47 



converged by it and meet at b b, where the diaphragm is 

 placed to intercept all the light except that required for 

 the formation of a perfect image ; the image at b b is 

 further magnified by the lens e e, as if it were an original 

 object. The triple achromatic combination constructed 

 on Mr. Lister's improved plan, although capable of trans- 

 mitting large angular pencils, and corrected as to its own 

 errors of spherical and chromatic aberration, would, in 

 some instances, be less effective without an eye-piece of 

 peculiar construction. 



The eye-piece, which up to this time is considered to be 

 the best to employ with achromatic object-glasses, to the 

 performance of which it is desired to give the greatest 

 possible effect, is described by Mr. Cornelius Varley, in the 

 fifty-first volume of the Transactions of the Society 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 refractions at two 

 glasses instead of one, and of increasing the field of view. 

 It consists of two plano-convex 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 chromatic aberration. Let fig. 32 represent the 

 Huyghenian eye-piece of a microscope, // 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, corres- 

 ponding with the colours of the prismatic spectrum. This 

 order of colours is the reverse of that of the common com- 

 pound microscope, in which the single object-glass projects 

 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 ^e given, and is called over -correcting the object-glass 



