M. C, Lea on a Theory of Photo-chemistry. 71 
A simple Savart polariscope is sufficient for making the obser- 
vations. In positing Brewster's neutral point, I have usually 
raised the lower sash of an attic window so that the bottom of 
the sash will screen the sun from the polariscope. I have thus 
been able, in every instance when the atmospheric conditions 
seemed favorable, to see very distinctly the neutral point, and 
the oppositely polarized bands above and below. 
Art. IX.— Contributions toward a Theory of Photo-chemistry ; by 
M. Cargy Lega. 
IN a somewhat extended series of experiments published at 
various times,* I endeavored to fix, as far as I was able, some 
of the facts of photo-chemistry, and more especially the nature 
of the action of light upon iodid of silver, at once the most im- 
portant and the most difficult of explanation of all the facts of 
photo-chemistry which fall under our notice. The phenomena 
exhibited by iodid of silver, in the point of view which they 
me to me, are the key to the whole matter, and pon 
es I propose to offer some theoretical views upon the general 
subject. 
and conduction. The loss of heat in this way where the body 
‘ected is much above the temperature of those that surround 
4€ phenomena of phosphorescence received an immense exten- 
Sion from the pap na ee beautiful experiments of Becquerel, 
