CHEMICAL RAYS. 241 



thing as absolute coldness in our corner of Nature the 

 atoms are always in a state of vibration, their vibrations 

 being executed to and fro across their positions of equilib- 

 rium. 



Into a vapor thus constituted, we have now to pour a 

 beam of light ; which most of you know to be a train of 

 minute waves, excited in, and propagated through, an al- 

 most infinitely attenuated and elastic medium, which fills 

 all space, and which we name the ether. It is hardly neces- 

 sary to remind you that these waves of light are not all of 

 the same size ; that some of them are much longer and 

 higher than others ; that the short waves and the long ones 

 move with the same rapidity through space, just as short 

 and long waves of sound travel with the same rapidity 

 through air, and that, therefore, the shorter waves must fol- 

 low each other in quicker succession than the longer ones ; 

 that the different rapidities with which the waves of light 

 impinge upon the retina, or optic nerve, give rise in con- 

 sciousness to differences of color / that there are, moreover, 

 numberless waves emitted by the sun and other luminous 

 bodies which reach the retina, but which are incompetent 

 to excite the sensation of light ; for, if the lengths of the 

 waves exceed a certain limit, or if they fall short of a cer- 

 tain other limit, they cannot generate vision. And it is to 

 be particularly borne in mind, that the capacity to excite 

 vision does not depend so much on the strength of the 

 waves as on their periods of recurrence. I have, as many 

 of you know, permitted waves to enter my own eye, which, 

 if their energy were that of light, would have instantly and 

 utterly ruined the optic nerve, but which failed to produce 

 any impression whatever upon consciousness, because their 

 periods were not those competent to excite the retina. 



The elements of all the conceptions with which we shall 

 have subsequently to deal are now in your possession. And 

 you will observe that, though we are speaking of things 

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