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LIGHT: COLOUR 291 Fio. 158.—Trough prisms by what is called a polyprism, of glasses ranging from crown to double-dense flint; but a cheaper and equally effective apparatus can be obtained of glass plates cemented together as in fig. 158, the outer strips, forming a V of about 60°, being about 6 inches long by 2 inches wide. They may be ceni( nted together with marine glue, in which case water, salt and water, and sugar of lead and water, will give considerable differences in disper- sion ; but by using glue or isinglass mixed with bichro- mate of potash or chrome alum, and then exposed well to sunlight so as to become insoluble, they may be filled with water, any medium solution, and carbon disulphide, or monobromonaphthalene, which is equally dispersive and less volatile. 166. Achromatism, and Compound Prisms.—That refrac- tion and dispersion are not necessarily proportional, is very readily shown by a prism of dense flint glass, of an angle which exactly neutralises the colour produced by the trough of water, when its refracting angle is placed the reverse way, or at the top. It will be seen that there is still left a very considerable amount of refraction. It will be evident that the reverse may also be done, and the refraction might be neutralised while still leaving prismatic dispersion; and both phenomena are excellently illustrated by a very convenient apparatus devised by Professor Weinhold and shown in fig. 159, of one third the real size, fig. 160 giving the details more clearly. One of the rods on the stand carries by the screw k a flint-glass prism of 20° angle, and the other by another screw Jc two crown-glass prisms of about 26° and 45°