ACTION OF DARK RADIATIONS. 203 



flash of light. There is a nerve specially devoted to the purposes of 

 vision which comes from the brain to the back of the eye, and there 

 divides into fine filaments, which are woven together to a kind of screen 

 called the retina. The retina can be excited in various ways so as to 

 produce the consciousness of light : it may, as we have seen, be excited 

 by the rude mechanical action of a blow imparted to the eye. 



There is no spontaneous creation of light by the healthy eye. To 

 excite vision the retina must be affected by something coming from 

 without. What is that something ? In some way or other luminous 

 bodies have the power of affecting the retina but how ? 



It was long supposed that from such bodies issued, with inconceiv- 

 able rapidity, an inconceivably fine matter, which flew through space, 

 passed through the pores supposed to exist in the humors of the eye, 

 reached the retina behind, and, by their shock against the retina, aroused 

 the sensation of light. This theory, which was supported by the great- 

 est men, among others by Sir Isaac Newton, was found competent to 

 explain a great number of the phenomena of light, but it was not 

 found competent to explain a?? the phenomena. As the skill and knowl- 

 edge of experimenters increased, large classes of facts were revealed 

 which could only be explained by assuming that light was produced, 

 not by a fine matter flying through space and hitting the retina, but 

 by the shock of minute waves against the retina. 



Dip your finger into a basin of water, and cause it to quiver rapidly 

 to and fro. From the point of disturbance issue small ripples which 

 are carried forward by the water, and which finally strike the basin. 

 Here, in the vibrating finger, you have a source of agitation ; in the 

 water you have a vehicle through which the finger's motion is trans- 

 mitted, and you have finally the side of the basin which receives the 

 shock of the little waves. 



In like manner, according to the wave-theory of light, you have a 

 source of agitation in the vibrating atoms, or smallest particles, of the 

 luminous body ; you have a vehicle of transmission in a substance which 

 is supposed to fill all space, and to be diffused through the humors of 

 the eye ; and, finally, you have the retina, which receives the successive 

 shocks of the waves. These shocks are supposed to produce the sen- 

 sation of light. We are her^ dealing, for the most part, with supposi- 

 tions and assumptions merely. We have never seen the atoms of a 

 luminous body, nor their motions. We have never seen the medium 

 which transmits their motions, nor the waves of that medium. How, 

 then, do we come to assume their existence ? 



Before such an idea could have taken any real root in the human 

 mind, it must have been well disciplined and prepared by observations 

 and calculations of ordinary wave - motion. It was necessary to 

 know how both water-waves and sound-waves are formed and propa- 

 gated. It was, above all things, necessary to know how waves, pass- 

 ing through the same medium, act upon each other. Thus disciplined, 



