[A) FRESNEL ON THE COLOURS PRODUCED IN 



ness, which gave nearly the same tint in the extraordinary image, 

 that is to say, the extreme red of the second order, or the purple 

 of the third. But on calculating from these data the appa- 

 rent rotation of the plane of polarization of the red rays in oil 

 of turpentine, on the theory of the apparatus which 1 have just 

 described, I found an angle more than double that which M. 

 Biot had determined by dii'ect measurement, and which he had 

 the goodness to communicate to me. To discover what could 

 occasion so great a difference, I wished to observe the series of 

 colours produced by different lengths (from to 50 centimetres) 

 of oil of turpentine. Having placed the tube in a vertical posi- 

 tion, and fixed the principal section of the rhomboid of calca- 

 reous spar in the primitive plane of polarization, I caused the 

 fluid which it contained gradually to flow out, and was very 

 much astonished to sec the extraordinary image pass through a 

 white slightly coloured, and finally arrive at black without show- 

 ing at all the red of the first order. 



It is sufficiently different from the red of the second order to 

 be easily distinguished; and by the simple inspection of the tints, 

 it is easy to observe that that which coi'responds to 50 centi- 

 metres of the oil of turpentine is not of the first order. Besides, 

 what still better determines its rank, is the thickness of the cry- 

 stallized lamina which causes the extraordinary image to vanish. 

 It may be objected, perhaps, that this disappearance only taking 

 place when the glass parallclopiped is used, it is possible that 

 double reflexion may alter the tint produced by oil of turpen- 

 tine, and cause it to descend in the order of the rings. But, 

 in the first place, on examining at the same time the direct 

 and the reflected images, one must be convinced that their co- 

 lour is absolutely the same ; secondly, experiment and theory 

 both show that double reflexion, at the incidence which produces 

 complete depolarization, modifies all the rays in the same man- 

 ner, and that, If it changes, in general, the interval which sepa- 

 rates two systems of waves polarized in contrary planes, this 

 change for each kind of rays is proportional to the length of their 

 waves ; so that it can neither raise nor lower the tint, the rank 

 of which solely depends upon the relation of the constant part 

 of the interval to the lengths of the different luminous waves. 

 Therefore it remains confirmed that the extraordinaiy image 

 passes from black to the red of the second order, without passing 

 throu2;h the red of the first. 



