THE MICROSCOPE STAND 



109 



is reflected, and the reflected light at the time it emerges has 

 travelled a path that is longer than the direct light by the 

 amount A B. This difficulty was overcome by making a special 

 pair of eyepieces whose focal points were different in position, 

 but it limits the use of the instrument to the use of special 

 eyepieces. 



Beck 



Fig. 102. — Diagram showing 



paths of light tlirough the Beck Binocular 

 Microscope. 



(M) Substage focussing adjustment. 



(N) Mirror. 



(0) Pillar. 



(P) Base. 



(x y) Object. 



(a/ y') Image formed by object glass. 



(x' y") Virtual image formed by eyepiece. 



(« «') Ramsden discs — conjugate images of 

 back equivalent plane of object 

 glass. 



