ofLtfi^ht; and a new Theory of Vision. '125 



whole of the medium, it is evident that none of the rays of light 

 would be suffered to pass through; and consequently we can only- 

 account for the transparency of a medium by supposing that it 

 is only partially covered by this fluid ; that is, onlv those parts 

 of it are covered w here the substance is to vvliich this fluid attaches 

 itself. Now, if there was no other substance to fill up those parts 

 on the surface of glass that this fluid did not cover, the surfaces 

 of the fluid would he rather spherical than flat (as water is when 

 it is only sprinkled on a surface, and does not entirely cover it), 

 or, at least, it vvould be l>ounded by sides; and in either case 

 would reflect light irregularly, and make the images of objects 

 appear confused. To prevent this, it is necessary that the whole 

 surface of the glass should be covered, that is, that the space left 

 vacant by this fluid should be filled up by the fluid that has the 

 property of refracting light; and tlieii, like all other fluids,it would 

 naturally level itself with the surface of the glass, and conse- 

 quently would reflect light only in such angles as would repre- 

 sent the images of objects distinct and unconfused. 



Here then is a necessity for the existence of such a fluid in ad- 

 dition to the probability : but it is also to be proved by the still 

 stronger evidence of positive fact; for the refracting substance, 

 as well as the reflecting substance, may be removed from the 

 surface of glass, and whenever it is, the power of refraction ?s 

 visibly destroyed. 



It has been supposed by Sir Isaac Newton and others, that 

 when light passes very obliquely to the further surface of glass, 

 none of the rays are suffered to pass through, but that all of theni 

 are reflected back, so that this surface then puts on the appear- 

 ance of quicksilver, which it docs not in any other instance. 



This piiicnomenou however is not produced by reflection, but 

 by rej'raclion; and the philosophers themselves acknowledge so 

 much, although by some strange oversight thev have jumbled 

 the two powers together, which in their effects are totally dis- 

 similar (except in tins instance, where it so happens that both of 

 them send the rays into the eye by the same angle), and have 

 agreed to call that reflection which by their own arguments is 

 produced by refraction. Maclaurin, in liis "Account of Sir Isaac 

 Newton's Discoveries," page 114, in speaking of gravity or at- 

 traction, says that " the rays of light on entering a medium are 

 constantly attracted towards the perpendicular, and when they 

 are incident upon the further surface of the glass, with a suffi- 

 cient obliquity, are all turned back into the glass, though there 

 be no sensible medium behind the glass to reflect it." This 

 phaenomenon then, even by their own account, is produced by 

 the rays of light being refracted or turned back by attraction in 

 the medium, and should be called refraction, which is produced 



Vol. 55. No. 206. June 1820. Q q by 



