touching his Theory of Light and Colours. 



199 



in a triangular glass prism O E F, that the rays A N that tend 

 out of the glass into air, do, 

 by inclining them more and 

 more to the refracting su- 

 perficies, emerge more and 

 more obliquely till they be 

 infinitely oblique, that is, in 

 a manner parallel to the su- 

 perficies, which happens 

 when the angle of incidence is about 40° ; and then if they be a 

 little more inclined, are all reflected, as at A v A, becoming, I 

 suppose, parallel to the superficies before they can get 

 through it. 



Let A B C D represent the rarer medium, E F H G the 

 Ai^ .B 



denser, C D F E the space between them or refracting phy- 

 sical superficies, in which the aether is of all intermediate de- 

 grees of density, from the rarest aether at C D to the densest 

 at E F ; A m n L a ray, A m its incident part, m n its incurva- 

 tion by the refracting superficies, and n L its emergent part. 

 Now if the ray A m be so much incurved as to become at its 

 emergence », as nearly as may be, parallel to C D, it is plain 

 that if that ray had been incident a little more obliquely, it must 

 have become parallel to C D before it had arrived at E F, 

 the further side of the refracting superficies, and so could have 

 got no nearer to E F, but must have turned back by further 

 incurvation, and been reflected as it is represented at A /*. v A : 

 and the like would have happened if the density of the sether 

 had further increased from E F to P Q, so that PQHG 

 might be a denser medium than EFHG was supposed; for 

 then the ray in passing from m to w, being so much incurved 

 as at n to become parallel to C D or PQ, it's impossible it 

 should ever get nearer to P Q, but must at n begin by further 

 incurvation to turn back, and so be reflected. And because 



