3io 



PAR-FOCAL OCULARS AND OBJECTIVES 



[Ch. IX 



"Jena, June 25th, 18S1. Dear Sir: The question which you ask 

 admits of a simple answer: In order to change the oculars of a micro- 

 scope without changing the focus of the objective, neither the dia- 

 phragm nor the field-lens must come to the same place in the micro- 

 cope tube, but the anterior (lower) focal points of the ocular systems 

 must do this. In the case of a Huygenian eye-piece, the said anterior 

 focus is a virtual one situated above the field-lens at a place D x , which 



is more distant from the field-lens than 

 the diaphragm D. The level of D* is 

 the place where the virtual image of 

 the diaphragm appears to an observer 

 looking through the field-lens. Rays 

 which are required to emerge from the 

 eye-lens as parallel rays (or nearly 

 parallel) must of course enter into the 

 ocular converging to the point D*. 

 Consequently, if different oculars are 

 inserted successively in such a way 

 that the point D* comes to the same 

 place of the tube always, the conju- 

 gate foci of object and image in the 

 objective remain unaltered. 



"This arrangement and no other one 

 fulfills at the same time the other re- 

 quest that the amplification of the mi- 

 croscope with different oculars should be in exact inverse proportion 

 of the equivalent focal length of the oculars. 



"The position of the point D x may be easily calculated for every 

 ocular. If a is the distance of the diaphragm from the field lens and 

 X the focal length of that lens, the distance of the focus D x above the 



a 

 diaphram (i.e. the distance from D to D*) will be: /3 = 



Virtual one** ^ 



Biajxlu-afiu 



ruu wo* 



Fig. 184. Par-focalization of 

 Oculars. 



(From the Microscopical Bulletin, 

 1886). 



X- 



a 



Hoping that these explanations will be found satisfactory for your 



aim, I remain 



Yours sincerely, 



Dr. E. Abbe. 



