LANTERN POLARISING APPARATUS 



339 



FIG. 185 



The other end of the elbow at B has a screw-collar, into 

 which screws the B collar of the optical front (fig. 94), with 

 stage, objective, and nozzle. Into this nozzle fits, so as to 

 rotate easily, an analyser, usually a Nicol prism. The whole 

 apparatus is shown in fig. 186, and is commonly known as 

 the lantern polariscope, or elbow polariscope. It is also 

 useful and convenient as a 

 table instrument for many 

 purposes, if a plate of finely- 

 ground glass is fitted into 

 the end which fits into the 

 lantern ; and will perform in 

 a most efficient and satisfac- 

 tory manner all ordinary ex- 

 periments, which do not re- 

 quire the rotation of the polariser, at a very moderate expense. 

 In using this instrument, it is placed on or in the flange- 

 nozzle with the elbow lying horizontally, so that the lantern 

 has to be deflected from the screen ; because the optical 

 portion of the instrument must be preserved in a horizontal 

 direction. To keep 

 it from turning 

 round in the flange 

 from its own weight, 



there is either a slot *ffl/ %^ 



fitting over a pin, ^ /sJO, 



or a simple bayonet 

 joint. 



The Nicol prism 

 analyser will per- 

 form all ordinary experiments, but its performance should 

 be examined. A Nicol of proper proportions will, with the 

 objective described in Chapter XII., just ' cover ' (i.e. give a 

 polarised field over) slides of the standard London pattern, 



z2 



186. Elbow Polariscopo 



