HOMOGENEOUS FLUIDS BY POLARIZED LIGHT. 63 



Making a similar calculation for the other kinds of rays, we ob- 

 I tain for the rotations which they suffer in traversing 1 centimetre 

 I of oil of turpentine, the following numbers : — 



Having fixed a lamina of sulphate of lime, 0™'"'46 in thick- 

 ness, upon a glass parallelopiped, I placed it at the extremity of 

 an apparatus filled with oil of turpentine, the length of which 

 1 could vary. By a double experiment I sought what length 

 produced the most exact compensation, and in what azimuth the 

 plane of double reflexion of the parallelopiped must be placed to 

 cause the complete disappearance of one of the images. The 

 length I found to be 2"" '03, and the azimuth about 35° to the left 

 of the plane of polarization ; it was the ordinary image which 

 disappeared. It follows, that to infer the rotation produced by 

 this tube, we must first deduct 90° — 35°, or 55°, from the 

 rotation which is produced by the lamina 0'"™'46 in thickness, 

 which is 1145°*8 for the mean red raj^s. We must, then, sub- 

 tract an entire number of half circumferences, depending likewise 

 upon the difference in the succession of the tints produced by 

 the lamina and by the oil of turpentine. My apparatus not per- 

 mitting me to follow them from C^'SO to 2'"-03, I calculated this 

 number from the preceding experiment, being sure that I could 

 not be half a circumference in error, and I saw that it was 

 necessary to deduct three half-circumferences, or 540°. The 

 rotation of the red rays produced by traversing 2'^*03 of oil of 

 turpentine is therefore 550°*8 ; dividing this quantity by 203, we 

 have for the rotation of the red rays in 1 centimeti'e 2°' 71 *. 

 This result accords very closely with that obtained by M. Biot 

 by the actual measurement of the angles, at least if they are the 

 mean red rays which predominate in the light that he employed. 



* Starting from these data, we find that the ordinary and extraordinary red 

 rays only differ in their velocity by -tt^,77^, and the ordinary and extraordi- 

 nary violet rays by - ; so that the double refraction of the red rays is 

 to the double refraction of the violet rays, as 1 to 1-34. 



