VOL XC.] PHILOSOPHICAL TRANSACTIONS. 6l5 



body striking a smaller one, propels it, without losing all its motion: thus, the 

 particles of a denser stratum of ether do not impart the whole of their motion to 

 a rarer, but, in their effort to proceed, they are recalled by the attraction of the 

 refracting substance with equal force; and thus a reflection is always secondarily 

 produced, when the rays of light pass from a denser to a rarer stratum. Let ab, 

 pi. 9. fig. 29, be a ray of light falling on the reflecting surface fg; cd the direc- 

 tion of the vibration, pulse, impression, or condensation. 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 dih or cif; and the angle 

 of reflection to that of incidence. Let pg, 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 sub-duplicate 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 refrac- 

 tion 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. 31, 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 atmosphere 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 pub- 

 lication on the Inflection of Light. It has already been conjectured by Euler, that 

 the colours of light consist in the different frequency of the vibrations of the lumi- 

 nous ether: it does not appear that he has supported this opinion by any argument; 

 but it is strongly confirmed, 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 motion the velocity of the corpuscles of light, and thus pro- 

 ducing the fits of transmission and reflection; and even this supposition does not 

 much assist the explanation. It appears, from the accurate analysis of the pheno- 

 mena which Newton has given, and which has by no means been superseded by 

 any later observations, that the same colour recurs whenever the thickness answers 

 to the terms of an arithmetical progression. Now this is precisely similar to the 

 production of the same sound, by means of a uniform blast, from organ-pipes 

 which are different multiples of the same length. Supposing white light to be a 

 continued impulse or stream of luminous ether, it may be conceived to act on the 

 plates as a blast of air does on the organ-pipes, and to produce vibrations regulated 

 in frequency by the length of the lines which are terminated by the two refracting 

 surfaces. It may be objected that, to complete the analogy, there should be tubes, 

 to answer to the organ-pipes: but the tube of an organ-pipe is only necessary to 



