52 FRESNEL ON THE COLOURS PRODUCED IN 



preceding memoir in explanation of the colours produced by 

 crystalline laminae placed between two glass parallelopipeds at 

 right angles to each other. It is natural to think, from the inti- 

 mate relation which exists between these two classes of phae- 

 nomena, that they result from the same general modifications 

 communicated to the luminous rays, and that the difference 

 which they present in the succession of the colours is alone due 

 to the double refraction not being the same for the different rays 

 in the fluid particles, whereas, on the contrary, it is sensibly 

 the same in the crystalline lamina. 



It is evident that the cause of the pha?nomena of colorization 

 to which they give rise must be sought for in the individual 

 constitution of the particles, as they are entirely independent of 

 their arrangement, and yet at the same time so dependent upon 

 their form, that, to use the expression of M. Biot, according to 

 the nature of the fluid the light is turned from left to right, or 

 from right to left. I shall suppose therefore that they are so 

 constituted as to produce in the luminous rays which traverse 

 them the modifications which they undergo in the apparatus 

 that I have just described ; that is to say, that the light on enter- 

 ing and on leaving each paiticle undergoes the same modifica- 

 tion as that produced by double total reflexion, and that it suf- 

 fers, besides, double refraction within it. 



I shall at first show, as the result of this hypothesis, that the 

 rays which have been ordinarily or extraordinarily refracted in a 

 particle thus constituted always suffer the same refraction in the 

 particles of the same nature which they successively traverse, 

 whatever may be the azimuths of their axes. 



Let OO' be the principal section of the 

 first particle, RR' and TT' the two planes 

 which correspond to those of double re- 

 flexion in the apparatus, and which I shall 

 call the plane of entrance and the plane of 

 exit', these are, by hypothesis, perpen- 

 dicular to each other, and inclined at an 

 angle of 45° to the principal section. Let 



0,0'i be the principal section of the second particle traversed by 

 the pencil of light, R, R', and T,T', the two planes in which it 

 suffers, at its entrance and exit, the modification just spoken of. 

 It consists, as was seen in the foregoing memoir, in each pencil 



