LIGHT: COLOUR 



293 



on end in the patli of the beam, the effects of each prism 

 singly being first shown. 



Compound fluid prisms are very useful for many experi- 

 ments with the spectrum. Fig. 161 is the usual form, the 

 space B being filled with carbon disulphide, or monobromo- 

 naphthalene, or phenyl-thiocarbimide, and the ends G, of 

 light crown glass. Similar prisms of smaller size may be 



PIG. 161 



FIG. 162 



constructed with the new highly- dispersive Schott glass instead 

 of fluid ; and such a prism fixed in a mount at the crossing 

 of the rays from the optical objective, is exceedingly con- 

 venient for many purposes. Another useful compound prism 

 may be made on the general principle of fig. 162, a single crown 

 prism of light glass, G, separating two cells B of dispersive 

 fluid : such a prism may be made of nearly double the disper- 

 sion of a prism bottle, with only the deviation of one, and 



Fio. 163. Wernicke's Prism 



with much less loss of light ; or it may be made like Thollon's, 

 with one cell of fluid only, of greater angle. The most 

 generally useful in projection is a direct prism, which for all 

 simple and rough experiments saves deflecting the lantern. 



Another improved form of direct prism for projection is 

 shown in fig. 163, as constructed by Messrs. E. & J. Beck for 

 Professor S. P. Thompson, according to a method suggestsd 

 by Wernicke. Cinnamic ether has the same mean refractive 



