ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



723 



temperature variations, either of the room or of the water-jacket, are 

 practically without influence, since both fluids are equally affected. On 

 the other hand the method offers the advantage of measuring these 

 effects with the greatest accuracy. For measurement it is not necessary 

 that the observation tube should be set in a rigorously invariable position 

 with regard to the double prism. The foregoing peculiarities seem to 

 suggest the especial adaptability of the instrument for many kinds of 

 scientific and technical investigations, such as the determination of 



Fig. 196. 



alcohol in alcoholic solutions, the degree of concentration of saline 

 solutions, &c. 



The instrument is made in two models. Model i. is for ordinary, 

 and Model ii. for more highly accurate (but slower) determinations. 



Fig. 197 shows diagrammatically the double prism of Model i. ■ The 

 prism is arranged by placing in a hollow metal cylinder a parallel-sided 

 plane glass plate diagonally, so that it is inclined at 45° to the glass end 

 planes. Thus we have a sort of glass bos divided into two independent 



