CH. XV] 



EXPERIMENTS WITH POLARIZED LIGHT 



625 



Another method of producing polarized light is to reflect the 

 light from a pile of glass plates (lantern-slide cover-glasses will 

 answer). At an angle of incidence of about 57 from the normal 

 as shown in figure 351, the light reflected from the glass surfaces 

 will be plane polarized. The specimen is placed at S, the objective 

 at O, and the analyzer N 2 , at the crossing of the rays as before 

 (fig. 351). The heating effect of the light reflected from the glass 

 surface is so small that a water-cell is unnecessary. 



881. Converging polarized light to show rings and brushes. 

 If the polarized light passing through a crystal is a converging 

 cone, the most beautiful phenomena are shown. Fig. 352 shows 

 the apparatus used to project rings and brushes. A parallel beam 

 of polarized light obtained as before, strikes the lens system Li, 

 designed to bring parallel light to a focus in a strongly convergent 

 beam. 1/2, renders the light again nearly parallel. LI, is the usual 

 form for this work and L 2 , is a substage condenser from a micro- 

 scope, either form will give good results if free from strain. The 

 objective O, is a lens of short focus and large diameter. It need not 

 be a special projection objective. Three objectives were tried 

 which gave good results, (i) A photographic objective, focus 

 12 cm., diameter 2 cm. (2) A magnifying glass, focus 6 cm., 

 diameter 4 cm. (3) A plano-convex lens, focus 5 cm., diameter 

 6 cm. The single plano-convex lens gave the best results except 

 that the figures were slightly distorted. The analyzing prism N 2> 

 must have a medium sized opening, 15 to 20 mm. free aperture, 

 otherwise it will cut off part of the field. 



FIG. 352. CONVERGING POLARIZED LIGHT TO SHOW RINGS AND BRUSHES. 



L, is the usual form of a lens system designed to bring parallel light to a 

 strongly convergent beam. 



L 2 is a microscope condenser, to parallelize the light from L t . 



objective to converge the light beam upon the Nicol prism (N 2 ), and to 

 focus the rings and.brushes on the screen. 



N 2 Nicol prism^with at least 1.5 to 2 cm. opening. 



