122 Transactions of the American Institute. 



surface boldly to the direct rays of a summer's sun, and. is not deflected 

 a hair's breadth by their brightest beam. 



Clearly, then, light is not a substance hurled at us in particles (no 

 matter how light and small) by the sun and other sources of light, 

 but must be a force of such a nature that its forward transmission 

 does not imply any projectile motion. 



Such a force as this we find in that sort of vibratory motion which 

 gives us the waves of the ocean, or of any other expanse of water, 

 and the fact that such waves do not involve in the forward progress 

 of their action, any forward motion or velocity of the particles con- 

 cerned, I will now show you by means of a very ingenious little piece 

 of apparatus contrived and used many years ago by Prof. O. N. 

 Kood.* 



You see upon the screen a number of little square blocks 

 arranged in the form of a wave, and which now, at a signal, rise and 

 fall in succession, so as'to give to the wave, of which they form the 

 profile, the appearance of advancing and receding ; and yet, if you 

 will fix your attention upon any one of these little blocks of light 

 you will see that it has no back or forward motion whatever, but 

 simply rises and falls. 



Such a wave could, therefore, " travel forward," as we say (i. e.), 

 have the successive up and down motion of particles, follow each other 

 with any imaginable velocity, and yet involve no forward motion or 

 impact of the particles whatever. 



Such, then, is our idea of light and of the manner in which it 

 travels about from place to place. 



We believe luminous sources to be, in fact, only vibrating bodies like 

 ringing bells, only with far quicker pulsations ; and from these centers 

 of motion we imagine waves, in successive rings, to run outward in 

 continuous flow in a rare, gaseous body, which we suppose to pervade 

 all space, and which we call the luminiferous ether. 



Let me illustrate this point to you in yet another way. I have here 

 in the vertical lantern, which I exhibited to some of you last win- 



* A magic lantern slide is prepared as follows : A grating A B (Fig. 1) of parallel bars is 

 first made, capable of covering the entire field. This may either be done by painting on glass, by 

 taking a " negative," blackened all over by " exposure " and "development," and scratching clear 

 lines through the " film ; " or, as I have found more convenient, by pasting narrow strips of paper on 

 one plate of glass and covering it with another. 



This double plate is inclosed in the usual way in a wooden frame, which has also a groove in which 

 can slide freely another long plate of glass on which is a transparent sinusoid. This also is best made 

 by cutting out a piece of dark colored crayon paper and pasting it on the strip of glass. 



When, as the figure shows, the sinusoid is over the grating, we see a number of separate parallelo- 

 grams arranged in a wave line ; but as the long strip of glass is moved over the grating the individual 

 parallelograms move up and down in succession, as in the progress of a wave ; and yet none of them, 

 by any possibility, having any advancing or lateral motion. 



