318 Dr. G. J. Stoney on Microscopic Vision. 



This is a fraction which the more deviates from unity the 

 greater r is, i. e. the more oblique the beam. Hence, the 

 more oblique beams, which bring out the finer detail, are 

 more increased in brightness than the less inclined, which 

 deal with the larger features of the object. Hence 



Proposition 6. 



Mounting the object in a medium of extra high refractive 

 index will, cseteris paribus, increase the conspicuousness of the 

 finer detail to be seen upon it. 



Of course other factors, some of which may be even more 

 potent, have to be taken into consideration, such as the ratio 

 of the index of refraction of the object to that of the medium 

 in which it is mounted ; for the farther this ratio is from 

 unity, the more conspicuous do all the features of the object 

 become. 



19. Proposition 7. Optical Contact. — Another proposition 

 which is of use in interpreting the phenomena presented by 

 the microscope is a consequence of the condition of the aether 

 in the rare medium when light is totally reflected from a 

 surface separating a dense and a rare medium. What then 

 occurs has been investigated by Sir George Stokes, in his 

 masterly paper " On the Formation of the Central Spot of 

 Newton's Eings beyond the Critical Angle " (vol. ii. of 

 Stokes's Collected Papers, p. 56). It is therefore only 

 necessary here to enunciate the result of that investigation in 

 the form in which it explains optical events which the micro- 

 scopist has occasion to make use of. 



Normally, when a microscopic object is " mounted dry," 

 i. e. is situated in an atV-space between the slip and the 

 cover-glass, no rays from it can, while traversing the cover- 

 glass, be more inclined to the vertical than the u critical 

 angle." Now immersion objectives are specially designed to 

 admit rays that have passed upwards through the cover-glass 

 in more inclined directions. Accordingly, when an object 

 that is mounted dry is examined by an immersion objective, 

 what normally happens is that only a part of the aperture of 

 the objective is made use of. The event is, however, different 

 if the microscopic object is excessively close to the cover- 

 glass, owing to the phenomenon investigated by Sir George 

 Stokes. 



It follows from Sir George Stokes's investigation that when 

 a plane separates an optically dense from a rare medium, then 

 there is a very thin layer of the rare medium of which the 

 optical properties are peculiar. In cases of total reflexion, the 



