RESPECTING SOUND AND LIGHT. 



548 



one, propels it, without losing all its motion : 

 thus, the particles of a denser stratum of 

 ether do not impart the whole of their mo- 

 tion to a rarer, but, in their effort to proceed, 

 they are recalled by the attraction of the re- 

 fracting substance with equal force ; and 

 thus a reflection is always secondarily pro- 

 duced, when the rays of light pass from a 

 denser to a rarer stratum. Let AB, (Plate 4. 

 Fig. 32,) be a ray of light falling on the re- 

 flecting surface FG ; c d the direction of the 

 vibration, pulse, impression, or condensation. 

 When d comes to H, the impression will be, 

 either wholly or partly, reflected with the 

 $aroe 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 FG, (Fig. 33,) be a refract- 

 ing surface. The portion of the pulse IE, 

 which is travelling through the refracting 

 medium, will move with a greater or less ve- 

 locity in the subduplicate ratio of the densi- 

 ties, and HE ^vill be to KI in that ratio. But 

 HE is, to the radius IH, the sine of the an- 

 gle of refraction ; and KI that of the angle 

 of incidence. This explanation of refrac- 

 tion is nearly the same as that of Rizzetti and 

 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- 34,) being so much longer than KI, 

 that the ray first becomes parallel to FG, and 

 then, having to return through an equal di- 

 versity of media, is reflected in an equal an- 

 gle. When a ray of light passes near an in- 

 flecting 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 to the body is retarded, 

 and of course the whole ray isinflected towards 



the body, (Fig. 35.) It has already been con- 

 jectured, that the colours of light consist in 

 the different frequency of the vibrations of 

 the luminous ether : the opinion is strongly 

 confirmed, by the analogy between the co- 

 lours of a thin plate and the sounds of a se- 

 ries of organ pipes, which, indeed, Euler ad- 

 daces as an argument in favour of it, al- 

 though he states the phenomena very inac- 

 curately. The appearances of the colours of 

 thin plates require, in the Newtonian sys- 

 tem, a very complicated supposition,, of an 

 ether, anticipating by its motion the velo- 

 city of the corpuscles of light, and thus pro- 

 ducing the fits of transmission and reflection; 

 and even this supposition does not much as- 

 sist the explanation. It appears, from the 

 accurate analysis of the phenomena which 

 Newton has given, and which has by no 

 means been superseded by any later observa- 

 tions, that the same colour recurs, whenever 

 the thickness answers to the terms of an arith- 

 metical progression, and this effect appears 

 to be very nearly 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. The greatest 

 difficnhy in this system is, to explain the 

 different degree of refraction of differently 

 coloured light, and the separation of white 

 light in refraction : yet, considering how 

 imperfect the theory of elastic fluids still re- 

 mains, it cannot be expected that every cir- 

 cumstance should at once be clearly eluci- 

 dated.^ It may hereafter be considered, how 

 far the excellent experiments of Count Rum- 

 ford, which tend very greatly to weaken the 

 evidence of the modern doctrine of heat, 

 may be more or less favourable to one or the 

 other system of light and colours, 



