izo SCIENTIFIC APPARATUS. 



more double image prisms, with quartz plates intervening, and has 

 been used for studying the combination of colours, as well as for 

 the effects of coukurs degradees, first discussed experimentally, 

 although by another method, by Helmholtz. Another application 

 of rotatory polarisation consists in determining the amount of 

 rotation of the plane of polarisation due to a column of given 

 length of saccharine and other solutions. The instrument whereby 

 this, and consequently the strength of the solution, is determined, 

 is known as the saccharimeter. 



One of the most remarkable phenomena connected with double 

 refraction is that known as conical refraction. This phenomenon, 

 which was predicted by the late Sir W. Hamilton as a consequence 

 of the wave theory, was first actually observed by Lloyd. It 

 depends upon the fact that the wave surface, or locus of plane 

 waves diverging from a point within a crystal, presents certain 

 singular dimple -like points; at the bottom of the hollow the 

 tangents arrange themselves in a cone instead of a plane ; while- 

 on the elevated parts in the neighbourhood of these points the 

 tangent plane meets the surface in a ring. Rays which have 

 traversed the crystal in the direction of one of these points emerge 

 in a cone ; while .those which have traversed it in a cone, so as to 

 meet one of the rings, emerge in a cylinder of parallel rays. The 

 first of these cases is called external, the second internal conical 

 refraction. For a long time arragonite was the only crystal used 

 for the study of conical refraction; but recently M. Nodot has 

 shown that crystals of sugar, bichromate of potash, and tartaric 

 acid will serve equally well for the purpose. 



Some substances possess the property, first investigated by Pro- 

 fessor G. G. Stokes, of apparently changing the colour of light, or, 

 more strictly speaking, of giving out radiations of a different period 

 of vibration from those which they received. The change is usually 

 from a colour of a higher to a colour of a lower degree of refrangi- 

 bility. Uranium, often used as an ingredient in the manufacture 

 of glass, gives out a canary colour ; sulphate of quinine a blue 



