Substances of High Refractive Power. 24:7 



colloidal to a crystalline condition, especially during the cold 

 weather, although I had previously exposed it to a tempera- 

 ture of —20° without causing any alteration beyond an in- 

 crease of viscosity. 



I would also ask permission to show the Society a speci- 

 men of another highly refractive substance, viz., metacinna- 

 mene. It is a polymeric form of cinnamene, obtained by the 

 action of light or heat upon the latter substance. 



Cinnamene is a colourless liquid, obtained by distillation 

 from the resin storax. Its refractive index I found to be 

 1*54 for the D line, nearly the same as that of Canada balsam. 

 When this liquid is exposed to light for a few weeks, or 

 heated to 190° in a sealed tube for half an hour, it becomes 

 a glass-like solid, contracting greatly during the change. 



In order to determine its refractive index, it was heated 

 until it became viscous and placed in a hollow prism. The 

 angle of minimum deviation for yellow sodium-light was 

 found to be 39° 44' 35"; whence its refractive index is cal- 

 culated to be 1*593. This is much higher than that of any 

 other resin, so far as I can make out; and metacinnamene 

 would make a very valuable cement, since it is readily 

 softened by heat and becomes remarkably hard and tough on 

 cooling, but it does not show any great adhesiveness for 

 glass. I hope to get good results by mixing it with other 

 things, such as the ketone above described. 



Another highly refractive organic substance is monobromo- 

 naphthalene, of which I exhibit a specimen. 



It is a nearly colourless liquid, more stable than similar 

 compounds containing bromine usually are, and boiling at 

 the high temperature of 285° C. Its refractive index for the 

 D line is 1*662; very nearly equal to that of naphthyl-phenyl- 

 ketone. I am rather surprised that it has not come into use 

 as a substitute for carbon disulphide for filling prisms, as it 

 is much less volatile and inflammable than the latter sub- 

 stance, while it has an even higher refractive and dispersive 

 power. 



The great desideratum at present is, a substance which has all 

 the excellent qualities of Canada balsam — colourless, neutral, 

 permanent in the air, becoming fluid when moderately heated, 

 but hard and tough when cold, and with a refractive index 

 of at least 1'66. 



Such a substance would not only be of great use in the 

 construction of polarizing and other prisms, but it would also 

 be invaluable as a medium for mounting microscopic objects; 

 since details of structure are brought out much more clearly 



S 2 



