294 OPTICAL PROJECTION 



index as one of the new Jena glasses, but is widely different 

 for the blue and red ends of the spectrum. A prism is there- 

 fore constructed by placing a very wide-angled prism F of the 

 glass, in a cell of cinnamic ether c c, closed by rectangular 

 glass plates. The yellow ray is undeflected, but the red rays 

 B and violet rays v are dispersed by the second face of the 

 prism F as well as by the first, as shown in the diagram. 

 This prism gives very great dispersion, and much better 

 definition than carbon disulphide, while the rectangular ends 

 are an advantage. The cost of one not quite 4 inches long is 

 about 61. 10s. 



167. Transparency Reciprocal with Reflection and Re- 

 fraction. It is interesting to show by experiment that in 

 proportion as we abolish reflection and refraction, by approxi- 

 mating the refractive indices of the portions of a mass, and of 

 the surrounding medium, it becomes more transparent, or less 

 visible. A screen of blotting-paper may be arranged between 

 two lanterns, one on each side, and a portion of it either 

 wetted or oiled. It will be seen that the treated portion 

 appears darker, or reflects less light, to a spectator on the 

 same side as the lantern illuminating it ; while if this be 

 darkened and the screen illuminated on the other side, the 

 same portion appears brighter, because the light passes 

 through instead of being reflected. If we arrange a screen of 

 thin paper with a grease-spot of two inches diameter in the 

 centre, between the rays from the bare radiant in the lantern 

 and the most powerful gas-flame or oil lamp that can be pro- 

 cured, adjusting positions and distances so that the audience 

 can see when the surface appears evenly illuminated, we shall 

 have an experimental demonstration of Bunsen's photo- 

 meter. 



An elegant experiment with a mass of powdered glass will 

 make this still more clear. A bottle must be prepared like a 

 prism bottle, but with parallel sides ; or a fair result may be 

 got with one of the scent-bottles ground and polished to two 



