LANTERN POLARISING APPARATUS 34? 



re-reflected into a horizontal direction and polarised by 

 the pile P with a black glass at the bottom. This arrange- 

 ment can be made cheaply and readily of any size desired. 



It will be observed, that while the horizontal direction of 

 the beam from the lantern is preserved in the Delezenne 

 polarisers, it is necessarily deflected several inches to one 

 side of the axis of the flange-nozzle. In the first instruments 

 made on this principle, the beam was brought down below 

 the nozzle ; but this makes the apparatus rather deep, and 

 requires a deep case for packing. To avoid this the polariser 



FIG. 194. Direct Reflecting Polariscope 



was turned sideways ; but that was found awkward, and also 

 necessitated a large case. To meet these objections I advised 

 reversing the polariser so as to deflect the beam upwards, and 

 Messrs. Newton & Co. now construct their instruments as 

 in fig. 194. It will be seen that it is then rendered quite 

 compact, and requires no larger a case than the Nicpl prism 

 polariscope. All the front portion is precisely the same as in 

 the instrument shown in section in fig. 190. 



Such polarisers are liable to the same objection as the 

 elbow form, not being capable of rotating the polarised beam* 

 This can, however, be effected through a simple expedient 

 suggested by Professor S. P. Thompson. If we cause the po- 

 larised rays to pass through a mica quarter- wave plate ( 211) 



