418 Dr. Leeson on the Circular Polarization of Light 



missioned a friend to procure for me from Mons. Soleil of 

 Paris an apparatus made according to Biot's own directions. 

 Experiments made with that instrument confirmed the conclu- 

 sions I before arrived at, and I may add, that an examination 

 of the apparatus itself has in some degree suggested the pos- 

 sible sources of error. 



I do not of course positively assert that the observations of 

 others have been erroneous, although strongly inclined to sus- 

 pect they may have been misled. Nothing but an examination 

 of the same sample could indeed establish such conclusion. I 

 have never, however, been so fortunate as to obtain a solution 

 of sugar possessing a decided /^/-handed rotation ; and 

 although it is certainly possible that the oil of lemons or oil 

 of turpentine which I have examined may differ from that ex- 

 perimented on by others, still I have tried so many samples, 

 wari'anted genuine and procured from so many different 

 sources, that I can scarcely imagine it possible to procure sam- 

 ples, obtained from the same species of tree, possessed of oppo- 

 site rotating powers ; I may also remark, that although I have 

 observed* variations as respected the amount, I have never 

 noticed any variation as respected the direction of rotation in 

 any of the samples of the same description of fluid*. 



Misunderstanding may perhaps result from the converse use 

 of the terms right- and /^il-handed rotation by different writers, 

 but this cannot affect the essential fact, if true, that oil of 

 lemons, for instance^ has an opposite rotation to that possessed 

 by oil of turpentine, or the still more interesting fact alleged, 

 unfortunately, I fear, incorrectly, that a solution of sugar as 

 existent in the juice of grapes rotates the plane of polarization 

 in an opposite direction to that in which it is rotated by a so- 

 lution of sugar obtained from the cane. 



The direction to which, in common with other writers, the 

 term right-handed is subsequently applied cannot be mistaken 

 by those who attend to the instructions hereafter given, and 1 

 would recommend every one, previously to experimenting on 

 fluid substances, to study the opposite appearances presented 

 on analysing polarized light when transmitted through speci- 

 mens of right- and left-handed quartz, as usually sold for il- 

 lustrating circular polarization. 



With a view to the more explicit understanding of the mode 

 in which the experiments referred to have been conducted, 

 and to assist those not conversant with the subject in repeat- 

 ing them for themselves, I shall very briefly explain what is 



* Since this paper was printed in the Memoirs of the Chemical Society 

 1 have obtained from Paris a specimen of "Laurel turpentine," as it was 

 termed, possessing lefi-handed rotation, 



