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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