128 Dr. Young's Experiments and Inquiries 



sation. When d comes to H, the impression will be, either 

 wholly or partly, reflected with the same velocity as it arrived, 

 and EH will be equal to DH ; the angle EIH to D1H or CIF ; 

 and the angle of reflection to that of incidence. Let FG, Fig. 

 30, be a refracting surface. The portion of the pulse IE, which 

 is travelling through the refracting medium, will move with a 

 greater or less velocity in the subduplicate ratio of the densities, 

 and HE will be to KI in that ratio. But HE is, to the radius 

 IH, the sine of the angle of refraction; and KI that of the angle 

 of incidence. This explanation of refraction is nearly the same 

 as that of Euler. The total reflection of a ray of light by a 

 refracting surface, is explicable in the same manner as its simple 

 refraction ; HE, Fig. 3 1 , being so much longer than KI, that the 

 ray first becomes parallel to FG, and then, having to return 

 through an equal diversity of media, is reflected in an equal 

 angle. When a ray of light passes near an inflecting body, 

 surrounded, as all bodies are supposed to be, with an atmo- 

 sphere of ether denser than the ether of the ambient air, the 

 part of the ray nearest the body is retarded, and of course the 

 whole ray inflected towards the body, Fig. 32. The repulsion of 

 inflected rays has been very ably controverted by Mr. Jordan, 

 the ingenious author of a late publication on the Inflection of 

 Light. It has already been conjectured by Euler, that the 

 colours of light consist in the different frequency of the vibra- 

 tions of the luminous ether : it does not appear that he has sup- 

 ported this opinion by any argument ; but it is strongly con- 

 firmed, by the analogy between the colours of a thin plate and 

 the sounds of a series of organ pipes. The phenomena of the 

 colours of thin plates require, in the Newtonian system, a 

 very complicated supposition, of an ether, anticipating by its 



