ON LIGHT. 225 



their images permanently on surfaces duly prepared to 

 receive them). Is there any physical mode of convey- 

 ance by which, reasoning from what we see in cases 

 which we are able to analyze, we can imagine either a 

 material agent to be bodily transported, or a movement 

 propagated, or an influence wafted, from place to place, 

 so as to render a rational and consistent account of the 

 phsenomena of light, or so at least as, generalized, and 

 (so to speak) sublimated in modes not inconsistent with 

 the known properties of matter, to do so 1 



(8.) One feature is common to all ordinary physical 

 modes of communication. The transmission from place 

 to place, be it of what it may of a letter by post, a 

 gunshot, a sound, a wave, a tremor, or a shock even of 

 an earthquake occupies time. It has a velocity : some- 

 times a very great, but anyhow a measurable one. Is 

 this the case with light ? The answer, from all ordinary 

 experience, would be in the negative. But this is only 

 because the velocity in question is so great that the 

 longest distances to which we can send a flash of light 

 and receive it back again by reflexion is traversed in an 

 interval of time too short to be perceived as an interval, 

 so that the reflexion appears to be simultaneous with the 

 direct flash. It is otherwise when we bring to bear on 

 the question the ingenious combinations and delicate 

 appliances of modern science. The telescope enables 

 us to become eye-witnesses in the way of astronomical 

 observation of events which take place at distances in 

 space almost inconceivably greater than any we can 

 measure here on earth; at times calculable beforehand, 



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