40 JOURNAL OF PROCEEDINGS. 
MEASURING THE WAVE LENGTHS OF LIGH’’ 
(Illustrated. ) 
Read before the Astronomical Section of the Hamilton Scien- 
tific Association, February 20th, 1903. 
BY C. A. CHANT, M. A., PH. D., LECTURER IN PHYSICS, 
TORONTO UNIVERSITY. 
At the present time there is no need to demonstrate the ex- 
istence of waves of light since the undulatory theory of light 
has been for many years considered as well established as the 
laws of universal gravitation. 
A principal characteristic of the phenomena attending 
wave-motion is what is known as interference. If a stone be 
thrown upon the quiet surface of water there spread from the 
centre, where it struck, ever-widening circles, alternate ones 
being elevated above and depressed below the ordinary level 
surface. If instead of throwing a stone some means be taken 
to keep up the disturbance of the surface, circular waves will 
continually spread from the centre, and as they reach any point 
of the surface the water there will be continually moved up and 
down with a harmonic motion. 
Suppose, now, waves be made to continually spread from 
a second centre as well, when the two sets of waves meet each 
passes along just as though the other were not there. But it 
will be easily seen that the point referred to the water may be 
elevated by both sets of waves, or it may be depressed by both 
sets ; or, again, it may be depressed by one set and elevated by 
the other, in which case there will be no motion at all. Here 
motion added to motion gives rest. If, further, we count the 
number of wave lengths from a point of rest to the two centres 
it will always be found that the difference of the two lengths is 
equal to an odd number of half wave-lengths, and if we can ac- 
