ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



725 



the sum of those produced by each double prism, and is in reality the 

 true deviation magnified four times. Fig. 199 is a perspective view of 

 this prism combination. 



Fig. 201. 



S 



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Fig. 201 gives a general view of the instrument. A is a tube for 

 admitting a stream of water of known temperature. 



If be the refractive angle of the prism (0 has been made 45°), 

 / the focus of the telescope measured in millimetres, 10 the value of a 

 'scale interval also in millimetres, A the scale reading, then the refractive 

 difference (A n) is given by 



A» = ^tan</> (Model i.) 



and 



X n 



2/ 

 w A 



*7 



tan 4> (Model ii.). 



