5 6 THE POPULAR SCIENCE MONTHLY. 



brightest color the hand points to XII., say noon. As the back Nicol 

 is turned round, say as the sun begins to sink, the color fades ; and 

 when the plate is turned so as to restore the color, the hand points to 

 I. Similarly, as the back Nicol is turned gradually farther, represent- 

 ing the passage of the sun westward during the afternoon, the position 

 of the plate giving the strongest color, as indicated by the hand, cor- 

 responds to the successive hours of the dial ; and when the Nicol has 

 been turned through 90, that is, when the sun has reached the horizon, 

 the hand has moved from XII. to VI. In this way, as its inventor has 

 remarked, a dial may be constructed which will work equally well in 

 sunshine or in shade, or even when the sun itself is overcast, provided 

 only that there be a patch of clear sky to the north. 



Up to this point we have reproduced in an experimental fashion 

 the general every-day phenomena, both celestial and terrestrial, which 

 give rise to polarization ; and we have given such general account of 

 them as will serve to connect them together, and to show that they 

 all belong to one system of laws affecting the nature of light. I 

 should, however, regret, and I feel confident that you would share in 

 that regret, if we were to leave the subject with its surface as it were 

 merely scratched, and without any attempt to penetrate deeper into 

 its substance. With your permission, therefore, we will devote such 

 time as you may be still willing to grant me to a few elementary ex- 

 periments in polarization, which, while certainly not less beautiful than 

 those which you have already seen, will, perhaps, better illustrate the 

 nature of the processes w T hich we are now trying to investigate. 



Polarized light, as indicated at the outset, is distinguished from 

 common light by the presence of certain peculiarities not ordinarily 

 found, and these peculiarities are to be detected only by means of 

 special instruments. Light which has been reflected or transmitted 

 at particular angles from various substances, light wdiich has been 

 scattered by small particles, is found to be in this peculiar condition. 

 So likewise is light which has passed through this transparent piece 

 of Iceland spar, or Nicol's prism, as it is called. Yet the light which 

 has so passed through, and which is now projected on the screen, is to 

 the unaided eye in no way different from the same light before its 

 passage. Nevertheless, if we examine or analyze it by means of a 

 second Nicol, we shall find the peculiarity of its condition revealed. 

 For if either of the Nicols be turned gradually round (and remember 

 that they are both transparent, colorless blocks of crystal) the light 

 gradually fades until, when it has been turned through a right angle, 

 the light is absolutely extinguished. On turning the Nicol farther the 

 light revives, and afterward again fades, in such a manner that in a 

 complete revolution the light is twice at its brightest, and twice is 

 extinguished. Now, light is due to extremely small and rapid vibra- 

 tions of a very subtle medium, which is snpposed to pervade all space. 

 The fact that vibrations (i. e., motions to and fro) in one direction can 



