IlillTS OF OPTICAL CAPACITY OP THE MICEOSCOPE. 423 



Or using equation (7) 





The last medium in front of the eye must necessarily he air, 

 therefore ?^i=l, and if we indicate hy a^ the angle of divergence of 

 the instrument measured in air according to Lister's method, then 



sin ao=n .sin a. Putting the amplification -^ = iV, then 

 ^^21 «^- sin ^a„ 



"With an amplification iV"^ by which the cone of light just fills the 

 pupillary opening, and which we shall call the normal amplification 

 of the instrument, K=Hq, Hence 



iVo = . sin ttc 



1^ 



(8) 



And if ao remains constant, 



B:E, = JV,' : N\ (8*) 



If as was assumed 



N> No 



Whilst ^=5; when i\^ £ iVo- 



That is to say. 



The hrightness of an image seen through the microscope is equal to 

 that of light filling the unoccupied eye^' when the amplification is less than 



* Dayhght is of course siapposed, and a monocular microscope in use. 



