Dr. G. J. Stoney on Microscopic Vision. 349 



asther within this layer is brought into a disturbed condition. 

 The disturbance in reality penetrates further down, but fades 

 out by a law so rapid that it is only sensible within a very 

 short distance (which depends on the wave-length) of the 

 plane separating the media. The layer of this small thickness, 

 within which the disturbance is sensible, may be called the 

 Stokes's Layer. 



It further follows from the investigation, that if light 

 emanates from a point within the Stokes's layer, it will be able 

 to pass up through the dense medium at angles that exceed 

 the critical angle. It is easy to verify this experimentally. 

 Take a glass prism — one of the pendants of a glass chandelier 

 is sufficient. Hold it at the distance of distinct vision from 

 the eye, and turn it till the light of the sky is seen like 

 a silvery sheen, totally reflected from the inside of one of its 

 faces. Then, without moving the prism or the eye ; press a 

 piece of chalk against the outside of that face. A small 

 portion of the chalk can thereby be brought so close to the 

 glass that the intervening chink is less than the thickness of 

 the Stokes's layer. This small portion of the chalk will then 

 be seen through the face of the prism, while the rest of the 

 chalk and the hand that holds it, which are beyond the Stokes's 

 layer, are quite unseen. The light from the chalk, by which 

 it is seen, has obviously passed through the glass at an angle 

 which is beyond the critical angle. Similarly : 



Proposition 7. 



If a microscopic object, mounted dry, is so close to the cover- 

 glass that the chink of air between it and the cover-glass is less 

 than the thickness of the Stokes's layer, then light from it can 

 pass up through the cover-glass and the oil above it, at angles 

 both within and beyond the critical angle, and may accordingly 

 reach any part of the front of an objective whose NA is more 

 than 1. 



20. With the help of these seven propositions, supple- 

 menting the more familiar laws of optics, nearly everything 

 in microscopic vision may be explained, and useful rules can 

 be deduced for the manipulation of the instrument. The next 

 part of this memoir will deal with applications of this kind. 



[To be continued.] 



