426 An Essay on the Reflection^ Refraction ^ and Inflection 



by attraction, — and not reflection, which is caused only by re- 

 pulsion. 



Light is reflected from the fmther surface of glass, as well 

 when the rays strike perpendicularly upon it as when they are 

 oblique : but this extraordinary reflection, as it is called, and 

 which by the way is only observable in a prism, is never visible 

 except when the ravs are passing out so obliquely that they would 

 be brought in again by the common law of refraction if there 

 was no reflection at all. It is impossible that the same cause 

 can produce different effects ; and it is therefore contrary to every 

 principle of sound philosophy to ascribe this phaenomenon to the 

 power of reflection, which involves so many difficulties, when it 

 may be produced by refraction without supposing any difficulty 

 whatever. 



It is evident that the same cause must produce the same effect 

 when all the circumstances are similar; and no reason can be 

 assigned why the first surface of glass should not give out as 

 much light as the other, except by supposing a different cause, 

 and that cause can only be refraction. I take it for granted, then, 

 that no one after this will attribute this extraordinary light on 

 the further surface of the prism to any other cause than that of 

 refraction ; and admitting this, it is easy to prove that the power 

 of refracting light is in a fluid that adheres to glass, and not in 

 the glass itself, because this fluid may be removed by pressure, 

 and the light, in that case, never makes its appearance. Upon 

 pressing my finger hard upon that part of the surface of the prism 

 that refracts the light into it again, the same phaenomenon is 

 exhibited that I before described, — the light is rnly visible in the 

 interstices between the lines of my finger ; and this cannot be 

 explained by supposing that the power of refracting light is in 

 the glass, and not in a fluid that surrounds it, because the glass 

 itself cannot be removed by pressure, and its attraction of light 

 must be just the same whether my finger press upon it or not. 

 Neither can it be inferred that the rays of light are intercepted 

 by the lines of my finger, because, if they can pass through the 

 closer texture of the glass, thev certainly can pass between my 

 finger and the glass ; and therefore it is impossible to account 

 for the phaenomenon, but by supposing that the absence of light, 

 on the pressure of mv finger, is caused by the removal of the 

 substance that refracts light in consequence of that pressure; and 

 as the glass itself cannot be removed, it is evident that the sub- 

 stance that has the power of refracting light is not the glass, but 

 a fluid that surrounds it. 



If then the facts I have stated be correct, and they are such 

 as may be very easily proved, my hypothesis of the reflection and 

 refraction of light is proved by as strong evidence as can be pro- 



. duced 



