JOURNAL AND PROCEEDINGS, 7% 
POLARIZED LIGHT. 
ead before the Astronomical Section of the Hamilton Scientific 
Association, December 18th, 1903. 
BY SPROR) CoA; (CHANT, IM. Aj. PH.D, 
The great essential fact which has been demonstrated in. inves- 
tigations into the nature of light is that the light-effect is due to 
definite periodic actions. Now, the simplest physical hypothesis we 
can suggest to account for the transmission of a periodic action is 
that it is due to a wave-motion. Hence we have the wave-theory of 
light, the substantial correctness of which no one properly informed 
now questions. 
The great method used to test for an undulatory motion is to 
apply the phenomenon of interference. If two wave-motions are 
travelling through the same space, or over the same surface of 
liquid, the motion of the substance at any particular point in the 
space will be the sum ot the two motions from the two sources, and 
it is possible for these two to be in opposite senses and so destroy 
each other’s effect; or the motions may be in the same sense, in 
which case the substance at the point will be moved through a 
double distance. When one motion is destroyed by another they 
are said to interfere, and this interference will occur when the waves 
are “out of step,” or the distances of the point under consideration 
from the two sources of the motion (which are supposed to be 
identical in behavior) differ by 1, 3, 5 or any odd number of half- 
wave lengths. The waves will be “ in step” and the action be 
intensified when this difference of path is 2, 4, 6 or any even num- 
ber of half-wave lengths, 7. e., any number of whole waves. 
We know that sound is due to wave-motion, and in this case 
the motion of the particles of the medium (usually the air) trans- 
mitting the sound are back and forth in the direction of propagation. 
For many years after the distinct promulgation by Huygens of the 
wave-theory of light it was thought that the ether, the medium 
transmitting the light, was gaseous in nature, and that the motions 
of its particles were longitudinal, or the same as in sound. 
