POLARIZED LIGHT. 99 



from most polished surfaces, such as a mahogany 

 table, a tea-tray, a piece of japanned leather, &c. 



During the earlier and later periods of the day, 

 the light reflected from that portion of the sky 

 opposite the sun is always polarized. 



It will thus be seen that polarized light is of 

 common occurrence, but the unassisted eye is un- 

 able to detect it, although one-half of the ordinary 

 beam is lost. "We may here remark that the loss 

 of light caused by various optical contrivances is 

 not usually detected by the eye. This is well 

 illustrated by the Binocular Microscope. Let an 

 object be examined with the tube directly over 

 the prism with the prism in position ; if we re- 

 move the eye for an instant, and withdraw the 

 prism, no difference will be detected, although in 

 the latter case double the amount of light has 

 been transmitted through the tube. 



This non-appreciation of an increase or diminu- 

 tion of light to the extent of 50 per cent, is per- 

 haps owing to the dilation and contraction of the 

 pupil of the eye. 



If the reflected and refracted beams of polar- 

 ized light are thrown simultaneously on a white 

 ceiling and a white screen, the spectator will 

 observe two spots of light of equal intensity. 

 A polarized ray may be obtained 



1. By reflection. 



2. simple refraction. 



3. double refraction. 



4. transmission through a plate of tour 



maline or crystal of herapathite. 



The diagram (fig. 16) on page 98 represents 

 the two first, c being the reflected, and d the 

 refracted ray. 



The polarization of a ray of ordinary light by 

 H 2 



