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 



