432 Dr. Leeson on the Ciradar Polarization of Light 



the depth of the liquid ; I would strongly advise any one ex- 

 perimenting on this subject to examine the appearances pre- 

 sented by pieces of unannealed glass, crystals, mica, selenite, 

 &c., by placing them upon the shelf K K when the apparatus 

 is adjusted. I should however observe, that some specimens 

 of unannealed glass, mica and selenite, possess circular pola- 

 rization as in quartz, and it probably exists to a much greater 

 extent than has been hitherto noticed. 



Water cooled below 39° is a curious example of the phse- 

 nomenon of depolarization last alluded to, which I discovered 

 about three years since, but did not then feel sufficiently satis- 

 fied with the mode of observation. I have now completely 

 verified the fact, which may be readily exhibited by means of 

 the apparatus now described. Since this paper was read I 

 have however obtained apparent evidence of rotation, amount- 

 ing to about 15 degrees for a depth of 15 inches, but rapidly 

 disappearing as the temperature of the water rises. I am 

 about repeating these experiments, and shall communicate the 

 result in a future paper*. 



When a Biot's apparatus, as constructed by Mons. Soleil, 

 is used, mistakes may arise, not only from the confusion of 

 the extraordinary and ordinary images revolving round each 

 other, before alluded to, but also from the graduation of the 

 circle, which is divided to 1 80° on either side of 0° and no 

 further. Supposing, for instance, a rotation of 79° to the 

 right, since on moving the vernier 1 1° to the left the obscura- 

 tion of O would be observed, it might be mistaken for a left- 

 handed rotation of 1 1°. Probably some such error occasioned 

 Soubeiran to state that a solution of sugar which showed an 

 original rotation of 71°} was altered after boiling as in the 

 following table : — 



Primitive syrup .... 71° right-handed. 

 After 20 hours .... 



... 25 11° left-handed. 



... 64. 



... 74 5° right-handed. 



Supposing Soubeiran mistaken as to the direction of the 

 rotation (and he tells us nothing of the succession of tints by 

 which it might have been determined), but correct as to the 

 points of obscuration, his statements would lead us to infer 

 that it was really a right-handed rotation throughout, but in- 

 creasing, as it naturally would do, from + 71° to + 95° as the 

 water evaporated. The difficulty of determining the exact 



• I have since discovered that this apparent circular polarization was 

 due to the effects of temperature upon the bottom of the glass vessel, a 

 circumstance which materially interferes with the delicacy and certainty of 

 such observations. I am still engaged in these experiments. Nov. 13, 1844. 



