36 



CONSTRUCTION OF THE MICROSCOPE. 



spherical and chromatic aberration, would, nevertheless, be of little 

 service without an eye-piece of peculiar construction. 



If we stopped here, we should convey a very imperfect idea of the 

 beautiful series of corrections effected by the eye-piece, and which were 

 first pointed out in detail in a paper on the subject, published by Mr. 

 Yarley, in the fifty-first volume of the Transactions of the Society of 

 Arts. The eye-piece in question was invented by Huyghens for tele- 

 scopes, with no other view than that of diminishing the spherical aber- 

 ration 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 ar- 

 rangement, accidentally cor- 

 rected a great part of the 

 achromatic aberration. Let 

 fig. 31 represent the Huyghe- 

 nian eye-piece of a micro- 

 scope,^ being the field-glass, 

 and ee the eye-glass, and 

 Imn 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 bb blue, and the 

 intermediate ones green, yel- 

 low, and so on, corresponding 

 with the colours of the pris- 

 matic spectrum. This order 

 of colours is the reverse of 

 that of the common compound 

 microscope, in which the sin- 

 gle object-glass projects the 

 red image beyond the blue. 

 The effect just described, of projecting the blue image beyond the 



