Dr. Gr. J. Stoney on Microscopic Vision, 



347 



That is, none of the luminous rulings which form the useful 

 part of standard image No. 2 — none of those that represent 

 any feature of the object, excluding those which produce 

 false effects like intercostal markings — are made any finer by 

 mounting the object in a medium of extra high refractive 

 index. But nevertheless an important effect is produced, viz. 

 that the finer of the rulings are made relatively brighter than 

 they were before, so that, cceteris paribus, the detail which 

 they portray becomes more conspicuous. 



This is evident from the accompanying diagram, in which 

 ah is the front of the objective and o the object. Both figures 



represent the course of one of the more oblique beams of 

 parallel waves from the whole surface of the object, the first 

 figure representing what occurs when the object is mounted in 

 a medium of the same refractive index as the cover-glass and 

 immersion oil, and the second figure representing what occurs 

 when the object is mounted in an optically denser medium. 

 Cceteris paribus, the ratio of the brightness of the beam that 

 reaches the objective in the two cases is as cos i/cos r, which 



1- 



sin 2 ?' x 



\ l-sin 2 >> / ' 

 where n and n' are the refractive indices in the two media, 



