i.j HOW BODIES BECOME VISIBLE. 3 



call Ether, and, according to the hypothesis, the vibrating mole- 

 cules of the luminous body impart their vibrations to the ether, 

 by which they are transmitted in the form of waves that travel 

 with an inconceivable velocity to the receiver. The mare quickly 

 the molecules are vibrating the greater will be the number of 

 the waves generated in the ether in unit time say one second ; 

 and as the velocity with which the waves are propagated is 

 the same, or at all events not widely different, for all kinds of 

 waves (about 186,000 miles a second) it follows that the length 

 of each wave will depend upon the rate of vibration of the 

 molecules of the luminous body. 



Turning now to receivers, we find they fall into three classes. 

 There is first that marvellous instrument the human eye. 

 There is next also a very marvellous thing the photographic 

 plate. Both of these are very largely used by the spectroscopist 

 and their importance cannot be overrated. And then, finally, 

 making up our third class of receivers, we have everything else 

 in nature. But although receivers of this class influence largely, 

 and are largely influenced by, the light they receive, they are of 

 comparatively small importance in spectroscopic study. 



In some such way as this, then, we may roughly image to 

 ourselves how bodies become visible to us, how they impress 

 themselves upon our consciousness through the eye. 



All bodies, whether far or near, are visible to us by means of 

 their unrest. No motion of particles, no light. If all the bodies 

 in space were absolutely tranquil we should never see them. 

 But the normal condition of everything in nature is a state of 

 most beautiful and exquisite unrest. Scientific men call this a 

 state of vibration ; but we need not quarrel about terms. Every- 

 thing in Nature, far or near, is in this state of unrest, and if it 

 were not so there would be for us no External World. From 

 every material substance, including all distant worlds, the 

 vibrations of their smallest particles or of their largest masses 

 come to us along a medium which scientific men call ether, not 



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