HOMOGENEOUS FLUIDS BY POLARIZED LIGHT. 51 



This succession of colours, so odd in appearance and so op- 

 posed to that obser\ed in reflected rings, may be explained in a 

 very simple manner, if we admit that the double refraction in 

 the oil of turpentine is not the same for rays of different kinds, 

 and that it is strongest for those whose waves are shortest. 

 It is known that the double refraction of the violet rays in cal- 

 careous spar is more marked than that of the red ; it is pro- 

 bably the same in other crystals ; but these differences are too 

 slight in relation to the difference of velocity between the or- 

 dinary and extraordinary ray. It is for this reason that we have 

 supposed until now that the interval which separates two systems 

 of waves was sensibly the same for rays of various colours. But 

 when the double refraction becomes extremely feeble, as in 

 oil of turpentine, where the velocities of the ordinary and extra- 

 ordinary rays scarcely differ by the one-millionth, it is verv pos- 

 sible that the dispersion of the double refraction (if I may so ex- 

 press myself) becomes a considerable part of the double refraction 

 itself. It would result from some approximative measurements, 

 to be mentioned in the sequel of this memoir, that the double 

 refraction of the extreme violet raj's ought to be about one and 

 a half that of the extreme red rays. This hypothesis does not 

 appear to me improbable, or even contrary to analogy, which 

 ought not properly to be stretched to its greatest length; and in 

 adopting it we are enabled to account for that singular anomaly 

 of which I have just spoken, and which without it appears to 

 me inexplicable. 



It is easily conceived that the interval between the two sy- 

 stems of waves being no longer the same for all the rays, as in 

 the phaenomenon of coloured rings, or in that presented by thin 

 crystalline laminae, but changing with the length of the lumi- 

 nous waves, the succession of the colours may be quite different, 

 as this interval is so much the greater in proportion as the waves 

 are short ; which alters doubly the relation between its length 

 and that of the luminous waves. Thus we arrive at the red of 

 the second order, when the interval between the two systems of 

 red waves has not yet exceeded that which would produce the 

 red of the first order, if it were the same in the rays of different 

 colours. 



This hypothesis enables us to apply to the polarization exer- 

 cised by homogeneous fluids, the theory which I set forth in the 



