THE HAMILTON ASSOCIATION. 4] 
curately measure these distances we can determine the length 
ci the waves producing them. 
One of the most convenient ways of producing effects such 
as described is to fasten to a prong of a tuning fork two light 
styles of glass which end just above the surface of mercury or 
other liquid. When the fork vibrates the ends of the styles 
act as centres of disturbance. (Several photographs of such 
effects were shown on the screen.) 
Similar results are producible by light. If light from a 
narrow source falls on two plane mirrors inclined at an angie 
a very little less than 180 degrees, each mirror will illuminate 
a part of the screen, and where these illuminations overlay in- 
terferences may be obtained (Fresnel’s two-mirror experi- 
ment). Or light may be passed through a properly shaped 
double prism so that one half is sent along a path slightly differ- 
ent from that of the other half, and where the two beams over- 
lap we may secure interference. (Fresnel’s bi-prism). An- 
other method is to allow light reflected from a small plane mir- 
ror to interfere with that incident directly on the screen. 
(Lloyd’s experiment.) [Photographs of these effects were 
exhibited on the screen.| Each of these methods may be used 
te determine the wave-length of the light. In each case it is 
shown that light added to light may produce darkness. 
The action of a lens is of a similar nature. Waves of 
light spreading from a point on passing through the lens are so 
altered in form that they run to a point (convergent lens) or 
spread out more rapidly than before (divergent lens). 
It is a common observation that water waves, on striking 
an obstacle, bend to some extent around behind it. So also in 
sound, an object placed between the ear and the sounding body 
does not entirely shut off the sound. Similar effects are ob- 
servable with light, but on a much smaller scale. If light be 
-admitted into . darkened room through a very small aperture, 
and an object be put in its path, the shadow will not be sharp. 
The light bends behind it and dies away gradually. For in- 
stance, if the light comes through a small round opening, and 
a circular disk be put in its path, there will be found in the cen- 
tre of the shadow, where one would naturally expect the in- 
