404 Messrs. Nichols and Snow on the Influence of 



lamp, diffusely reflected from the pigment, enters a rectan- 

 gular prism at B, where it suffers total reflexion and is thrown 



J 





A* 





o 





\> 



/v, 



Q_ 





P 



© 



Fig. 1. 



ID Av'/ /"/ 



® 



A 



vertically downwards. The rays then enter another prism, 

 identical in size and form with the first, which sends them 

 through the upper half of the slit of the spectroscope in a 

 direction parallel to the axis of the collimator-tube, S 0. 



In multiple circuit with the lamp I/, and supplied from the 

 same battery, is the precisely similar lamp L, which serves as 

 the reference standard. Rays from this lamp are first ren- 

 dered parallel by the lens of short focus A, and then, before 

 roaching the collimator, pass through the pair of Nicol prisms, 

 N and W, which act respectively as polarizer and analyser. 

 The light upon emerging from the analyser enters a pair of 

 reflecting prisms similar to those which have just been de- 

 scribed and symmetrically placed, and so through the lower 

 half of the slit into the collimator and along a path every- 

 where parallel to that followed by the light from the pigment. 



The disposition of the four right-angled prisms before the 

 slit is shown in fig. 2. This arrangement offers a certain 

 advantage over the usual devices for Fig. 2. 



introducing the rays of the com- 

 parison-light into the collimator-tube. 

 The adjacent edges of the two inner 

 prisms are in contact with the slit, 

 and when viewed through the eye- 

 piece they form a sharply defined 



boundary in the middle of the field. 



The result is that when two bundles 



of light are introduced by means of the device under con- 



