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SUMMARY OF CURRENT RESEARCHES RELATING TO 



is placed between the slide and cover-slip, and the under surface of the 

 slide is moistened with a drop of liquid of same refraction-index as the 

 glass. If the angle of incidence of the beam is suitably chosen, the 

 interior beam meets the cover-slip at the angle of total reflexion, and 

 throws no light on to the objective. Any ultra-microscopic particles 

 present in the liquid become, however, diffractive, and therefore self- 

 luminous. The effect on the objective is to render these bodies visible 

 on a dark ground. In the figure the angle of the parallelopiped was 

 about 51°. 



The authors consider that their method has the great advantage of 

 using a large percentage of the light emitted from the source. The 

 experiments must be conducted in a darkened room. A view of the 

 actual apparatus is given in fig. 38, where it will be noticed that the 

 light issuing from the condenser of a small inclined lantern is con- 



Fig. 38. 



-centrated by a lens on to the parallelopiped. The lamp on the left is 

 used when it is desired to view the liquid as a transparent object. 



The examination of Lippmann's films liquefied showed that the 

 ultra-microscopic particles of silver bromide are in a state of Brownian 

 movement. The authors suggest that this fact may have a bearing on 

 photography in colours. A thin solution of Chinese ink behaved 

 similarly. A preparation of ferrocyanide of copper was examined as a 

 specimen of a colloid, and highly exhibited the Brownian movement ; 

 but, when a minute quantity of alum solution was added, the motile 

 particles instantaneously disappeared, and granular masses of ordinary 

 precipitated ferrocyanide of copper were produced. The property 

 possessed by colloids of diffusing light is probably due to the presence 

 of very minute particles, and the authors think that their experiments 

 are very suggestive to biologists who wish to study the action of saline 



