413 M. Vevdet on the Optical Properties developed in 



The azimuth of the plane of polarization was always deter- 

 mined by observing the passage-tint {teinte de passage). When 

 the body under examination is colourless or faintly coloured, it 

 is known that the passage-tiut indicates the plane of polariza- 

 tion of the middle yellow rays ; but the case is quite different 

 with a strongly coloured body, and grave errors are incurred if, 

 to obtain the relation between the optical action of such a body 

 and that of water, we compare without correction the two devia- 

 tions of the passage-tint. This source of error was got rid of 



being set to work, I observed tlie following azimuths of the passage-tint : — 



50 38 

 50 44 



50 42 

 60^ 



Mean . . 60 41 

 The water being replaced by sulphate of nickel, the azimuths — 



61 2« 



51 23 

 51 20 

 51 2(J 



Mean . . 61 23 45" 

 were obtained. On reversing the current, the a/.lmutlis of the jiassagc- 

 tint from the sulphate of nickel were, — 



Mean . . 46 34 15" 

 Finally, the sulphate of nickel being replaced by distilled water, the last 

 scries of azimuths were observed: — 



Mean . . 47 12 30" 

 Hence we get the following values for the rotations : — 



Distilled water ... 1 44 15 



Sulphate of nickel . . 2 24 45 

 In order to deduce from these numbers the true relation between the 

 optical actions of suljjhate of nickel and distilled water, they must be sub- 

 mitted to two corrections which \\'\\\ be described in the sequel. 



It is to be observed, that in taking the mean of the azimuths derived 

 from distilled water, and the mean of those from the sulphate of nickel, 

 one ought in both cases to get the azimuth of the jirimitivc plane of polar- 

 ization, and that consequently, if the exjierimcnts have been well jicrformed, 

 these two mean values should be equal. Their actual values are 48° 56' 45" 

 and 48° 6f)', the difference between which comes within the possible errors 

 of observation. 



