THE POLAKIMETER 



71 



and we have thus an emergent polarised ray fg. In practice 

 the absorption of the ordinary ray is effected by mounting the 

 Nicol prism in a black mounting. Such a prism is known as 

 a polariser. A second similar prism placed parallel to the first 

 will of course allow the ray similarly to pass through ; if held 

 at right angles, on the other hand, it will act in a similar way 



FIG. 13. POLABIMETEB (Schmidt and Haensch pattern). 



to the cross tourmaline plate, i.e., as a hypothetical barred 

 screen, and will extinguish the ray. The second Nicol prism, 

 as it enables us to recognise the polarised ray emerging from 

 the polariser, is known as the analyser. An instrument fitted 

 with these two prisms, together with suitable lenses for observ- 

 ing the ray and with a tube between the prisms in which 

 substances can be placed to observe their effect on the polarised 

 ray, is known as a polarimeter (Fig. 13). Certain details of 



