346 M. G. Wertheim on the double Refraction 



alight on the exact tint to which his measurement of the thick- 

 ness of the plates refers. 



To operate with more exactitude, I have had recourse to the 

 homogeneous light emitted by salt and alcohol, which light, 

 according to Mr. Miller, contains almost solely orange rays in 

 the vicinity of the line D of Fraiinhofer, and the wave-length of 

 which is consequently 589 millionths of a millimetre. We shall 

 hence have extinction of the extraordinary image whenever the 



difference of the paths is an even multiple of ^ or of 294*5 ; 



and extinction of the ordinary image whenever the difference of 

 path amounts to an odd multiple of the same quantity ; we shall 

 also know the amount of the charges which produce this differ- 

 ence of path. It must, however, be remarked, that besides the 

 orange rays, the flame of alcohol and salt contains a certain 

 quantity of violet light ; hence in these experiments the black 

 colour, which would be the result of a complete extinction, is 

 replaced by a violet coloration, which is very distinct in one of 

 the two images, when all the orange rays have disappeared ; this, 

 however, does not interfere in the slightest degree with the ex- 

 actitude of the experiments, for it is easy to seize the moment 

 when this colour presents itself in the greatest purity and with 

 the least intensity. In this manner we obtain a certain number 

 of fixed points in the chromatic scale, which it is our purpose to 

 construct, and others are found with equal certainty by operating 

 with white light. 



M. Biot* assigns the arc of 24 degrees to the rotation 

 impressed upon the mean yellow rays by a plate of rock-crystal 

 of the thickness of one millimetre ; combining this determina- 

 tion with the rotations which refer to the lines of the spec- 

 trum, the length of whose undulations are known, we find for 



these rays the length 550*6, and consequently we have ~ =275*3 ; 



but we also know the limiting tints of the reflected and trans- 

 mitted rings ; they are those in which the mean yellow ray is 

 destroyed. We can therefore determine the charges which give 

 these teintes de passage, sufficiently sombre in the first rings at 

 least, and characterized with sufficient clearness to serve as points 

 of reference. Among these colours the teinte sensible distin- 

 guishes itself, which, according to M. Biotf, is reflected by a 

 plate of air of about 21 millionths of an English inch, or of 



* " Memoire sur les phenomenes rotatoires op^res dans le cristal de roche" 

 (M^. de VAcad. vol. xx. p. 284). 



t *• Memoire sur la polarisation lamellaire " {Mim. de VAcad. vol. xviii. 

 p. 644). 



