THE MICROSCOPE. 



27 



reverse of that indicated in Fig. 23 ; that is to say, the 

 violet rays V V are focalised at a greater distance from F 

 than the red rays R R. The 

 focus of E being, of course, 

 shorter for the violet than 

 for the red, these different 

 rays are focussed by E on 

 the same points of the retina, 

 and thus an image, colour- 

 less or nearly so, is seen. 

 The manner in which spheri- 

 cal aberration in the eye- 

 piece is prevented by adjust- 

 ing its lenses, so that the 

 spherical fault of the one 

 corrects that of the other, 

 has been already explained 



(31)- 



With most microscopes 

 eye-pieces of different powers 

 are supplied. Those of Eng- 

 lish microscopes are distin- 

 guished by letters A, B, 

 etc., while those of foreign 

 makers are distinguished by numbers i, 2, 3, and so on ; 

 the A of the former and the i of the latter having the 

 lowest magnifying power. The shorter the eye-piece, the 

 higher is its power. By many English writers a short is 

 termed a "deep" eye-piece; the reader has therefore to 

 remember that when " deep eye-piecing" is recommended by 

 some writers, the employment of an ocular of high magni- 

 fying power is indicated. As the confusion is perplexing 

 and altogether needless, the term "deep" ought to be 

 abandoned. 



Seeing that the eye-piece merely magnifies an image 

 produced by the objective, any fault in the image produced 

 by the latter is exaggerated. On this account, only very 

 perfect lenses can bear a shallow eye-piece, and therefore 

 its employment is of much service in testing the quality of 



FIG. 31. The eye-piece of Huyghens. 

 (Dippel, slightly altered.) 



