64 FRESNEL ON THE COLOURS PRODUCED IN 



Making the same calculation for the other rays, we obtain 

 the following angks : — 



Orange rays 



3-07 



Yellow rays .... 3*42 



Green rays .... 3'91 



Blue rays 4*44 



Indigo rays .... 4'87 



Violet rays . . . . 5*35 



These results differ sensibly from those deduced from the 

 previous experiment, and the bases of the calculation are in 

 fact sufficiently different; for if, by a proportion, setting out 

 from the data of the second observation, we inquire what length 

 of oil of turpentine ought to be exactly compensated by a lamina 

 of sulphate of lime corresponding to the number 21 of the first 

 column of Newton's table, we find it should be 0'^*548 instead 

 ofO'"50. 



Notwithstanding the difficulties which arise from the greater 

 length of the apparatus in the second experiment, and which 

 might be the causes of error, I am led to think that the results 

 which have been deduced from it are more exact than the former, 

 not only because the measurements and observations were made 

 upon larger quantities, but also because I attended more to the 

 precautions which are necessary to approach exactitude. Ne- 

 vertheless, I do not consider even these last results as very exact, 

 because the apparatus was not arranged in a sufficiently conve- 

 nient manner for making such delicate observations with preci- 

 sion*. Before having the honour of presenting them to the 

 Academy, I should have wished to have repeated the expe- 

 riments with a better-arranged apparatus, and to have verified 

 these angles by direct measurements of the rotation in homo- 

 geneous light ; but other researches oblige me to relinquish 

 these, at least for some time. 



I have shown how it is possible to distinguish the different 

 phgenomena presented by oil of turpentine, in supposing that 

 each of its particles possesses the power of double refraction, 



* It appeared to me that the tints produced by the 2°i03 of oil of tur- 

 pentine were a little less feeble than those of the lamina 0">*46. In traversing 

 2""60 of this essential oil, polarized light still presents an appreciable coloriza- 

 tion ; this appears to establish a slight difference between the phaenomena and 

 the hypothesis of complete compensation by the interposition of a lamina of 

 sulphate of lime. 



