436 An Essay on the Hejiection, Refraction) and Inflection 



be stationary ; while the other must change its place on the re- 

 tina, and of course will appear to be in motion. By the same 

 rule the moon, in stormy weather, very frequently appears to be 

 in motion ; whereas, if we fix our eyes steadfa?th- upon it, we per- 

 ceive at once that the moon is stationary, and that it is the clouds 

 that are in motion. Whenever we are moving, objects that are 

 motionless change their position with respect to one another; 

 and whichever of two objects we look directly at, will appear to 

 keep its position, and the other to be in motion; and upon the 

 same principle we may account for this phaenomenon. 



7. The reason why the images of objects appear to be fainter 

 in Iceland crystal than in glass is this : Whenever an object is 

 viewed through a plate of plain glass, there are very near as many 

 rays from that ol)ject enter the eye, as there are when the glass 

 is out of the way : but in all double refracting substances, half 

 the rays that would enter the eye under any other circumstances, 

 are bent out by the extraordinary refraction, the rays that do 

 enter the eve are so much the thinner, and the light in conse- 

 quence is proportionally fainter. 



It affords very strojig grounds for the presumption that an hy- 

 pothesis is true, when it accounts for plia'nomena by principles 

 that are either univerally received, or are known to exist in na- 

 ture. If pieces of glass, cut in the form of Iceland crystal, will 

 exhibit the phaenomena I have supposed in figures 3 and 5, (and 

 if they will not, the principles of refraction laid down by the 

 writers on optics cannot be true,) then the only difference be- 

 tween glass and Iceland crystal is, that the latter possesses a dou- 

 ble refracting power which the other has not ; and it surely is 

 more pliilosophical to su|)pose that this secondary power is owing 

 to a secondary substance, when the existence of a first substance 

 possessing the power of refraction has been fully proved, than 

 to imagine, with Newton, that the particles of light have two 

 sides of different power by which thev are differently refracted, 

 according as either side strikes against the medium ; or, with 

 Huygensjthat the waves of light are capable of moving the matter, 

 which produces the refraction in a medium, when it is in one 

 position, without having any such power when it is in another. 

 Newton accounted for the phaenonienon of double refraction by 

 supposing an original difference in the rays of lig'"' * '•" "" 

 of which " some rays are constantly refracted after the usual man- 

 ner, and others constantly after the uiuisual manner." But if the 

 phaenomenon of double refraction depended solely upon this dif- 

 ference of properties in the rays, the ph.'enomenon would be exhi- 

 bited in glass as well as in Iceland crystal, and in neither of them 



• He asks (Query 2o) Have not the rays of liglit several sides, endued 

 with several original properties ? 



with 



