310 



PAR-FOCAL OCULARS AND OBJECTIVES 



CCh. IX 



"Jena, June 2Sth, 1881. 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"', 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 dififerent 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 ampHfication of the mi- 

 croscope with diSerent oculars should be in exact inverse proportion 

 of the equivalent focal length of the oculars. 



"The position of the point D* 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* above the 



diaphram {i.e. the distance from D to D"^) will be: P = — 



X — a 



Hoping that these explanations will be found satisfactory for your 



aim, I remain 



Yours sincerely. 



Dr. E. Abbe." 



Fig. 184. 



JUW Ugf 



Par-focalization op 

 Oculars. 



(From the Microscopical Bulletin, 

 1886). 



