lxiii 



Light and heat are transmitted by the vibrations or undulations of a 

 fluid far more attenuated than the air or any known gas. This fluid fills 

 the illimitable regions of space, and is known by the name of the lumini- 

 ferous ether. By its wonderful tenuity it is able to pass almost without 

 hindrance through the molecular structure of the densest substances known. 

 To use the illustration of Thomas Young, who was chiefly instrumental 

 in establishing this theory, " this fluid passes through the solid matter of 

 the earth as a breeze does through a grove of trees." With our ideas of 

 the solid and compact structure of certain bodies, it is hard to compre- 

 hend the possibility of any fluid possessing such tenuity as to enable it to 

 pass almost instantaneously through glass, crystal, metals, and stones, 

 yet the creation of an etherial vacuum seems an utter impossibility ; while 

 the evidence of the existence of the luminiferous ether within the most 

 perfect atmospheric vacuum that man has yet devised, is absolutely irre- 

 fragable. Hence we must admit that the atoms of matter are not in 

 such immediate contact as to prevent the passage of this fluid through 

 them. 



The length of the waves of light, and their periods of vibration, 

 although so infinitely small and inconceivably rapid, have nevertheless 

 been accurately determined. They are known to be transmitted at the 

 rate of about one hundred and ninety thousand miles per second; or 

 from the sun to the earth in eight minutes. The length of the waves that 

 produce what are called the visible or colored, rays, have been accurately 

 ascertained. How this can be done I have not time now to explain; but 

 when it is remembered that the length of a wave of red light is only 

 the -ss^sjs part of an inch, and of a violet wave only the 57 J 7ff part of 

 an inch; and that 699,000,000,000,000 of the violet waves strike the re- 

 tina in a second of time, we can comprehend what a triumph of Science 

 is exhibited in ascertaining these minute facts so positively as to be capa- 

 ble of indubitable proof. Four hundred and seventy four-millions of 

 millions of the waves of red light enter the retina per second. Less than 

 this number of heat rays enter the eye per second, but the rapidity of 

 their vibration is not sufficient to excite vision. The chemical rays are 

 still more rapid, but because of their great rapidity the retina is not 

 affected by them. They are therefore invisible to us. 



When the ethereal waves fall too rapidly or too slowly upon the retina, 

 they fail to throw the atoms of the nerve tissue into periods of vibration 

 which coincide with them, and are. therefore, unable to excite vision ; just 

 as a powerful musical note fails to awaken into responsive vibration a 

 tuning-fork, a harp-string or other sonorous body whose vibratory periods 

 do not concur with it. 



All waves of light and heat travel at the same speed, hence the shorter 

 waves beat more rapidly upon the objects on which they impinge than 

 the longer waves. It must not be supposed, however, that the ether it- 

 self is moving at this immense speed. A sea-fowl floating upon the 

 surface of a lake rises and falls with each wave that rolls into the shore, 

 but the bird simply makes a slight movement to and from the shore with 



