Scientific Lectures. 123 



ter at the rooms of your Institute, a little circular tank or pond of 

 water, in the center of which, by means of a very neat contrivance of 

 Messrs. Hawkins & Wale, I can make, at will, single or successive 

 vibrations fit to be the origin of outspreading waves.* 



The image of all this you see upon the screen, and as I make a 

 single motion you see a single wave-ring run out from the center, 

 expanding until it strikes the sides of this tank, from which it is, in 

 turn, reflected back to the center. 



This, then, well represents the action of a light center and its radi- 

 ations, but perhaps I can make this more clear by another illustration. 



Some time since I stood in a large conservatory containing a great 

 pond or basin of water for aquatic plants. In the middle of this 

 played a fountain to freshen and renew the pool, and as the jet threw 

 every moment splashes of water into the basin, circles of ripples from 

 this center, ran widening outward over the smooth surface. Here 

 and there they met the stem or flower of some flag or lotus, and then 

 would make it tremble with like vibrations, and cause it to become a 

 new center of minuter undulations. In other places, masses of rock- 

 work rolled back the tiny waves and left quiet spaces in their rear, 

 which the rippling waves failed to reach or to disturb. 



At this moment, in this place, we have a parallel to the scene 

 which I have just described. The light-conveying ether which per- 

 vades this building (as it does all other places) stands for the water 

 of the green-house pool. Here in this lime light we have the foun- 

 tain of fluid which is rippling the surrounding ether (at this moment 

 a powerful lime light, which had been arranged under a screen upon 

 a stand in the middle of the stage, was uncovered and emitted a blaze 

 of light which filled the entire building), and sending wavelets of 

 light outward in every direction. These waves strike on many 

 objects and cause them to become centers from which emanate like 

 luminous motions, which, passing to our eyes, convey to them the 

 impressions of light and color. Here and there solid masses throw 

 back the light-producing waves, and protect spaces in their rear from 

 these disturbances, and give us thus the quiet of the shadows ; and 

 if one point more were needed to complete the parallel, I find it 

 when, looking around, I see on all sides no unapt representatives of 

 the flowers of the conservatory. 



* The apparatus for producing the waves is as follows : A little brass box A (Fig 2), with a 

 solid base B, is covered on top with sheet rubber, and has a small tube C D (a common mouth blow- 

 pipe, in fact), running out from its side. If this is held so as to bring the point D over the spot 

 where a wave is to be developed, and the rubber cover gently tapped with the finger, each tap 

 causes a puff of air to escape from D, and this produces a well defined and single wave. 



