378 



rough experiments, made one evening (about the month 

 of July, 1837) in company with Mr. Grubb, before I had 

 received the instrument from his hands, and merely vpith 

 the view of showing him, when it was finished, the kind of 

 phenomena that I proposed to observe with it, and the mode 

 of observing them. But the instrument w^as so far superior 

 (in workmanship at least) to any apparatus previously em- 

 ployed for this sort of experiments, that it was impossible, 

 without great negligence in using it, not to obtain measures 

 of considerable accuracy. I did not, however, at the time, 

 set much value on these measures, because I expected shortly 

 to possess a series of observations made with every possible 

 precaution ; but having chanced to preserve the paper on 

 which they were noted down, I was tempted, a few days ago, 

 to try how far they agreed with my formulae ; and the agree- 

 ment turns out to be so close, that I think myself justified in 

 publishing them. Besides, it will be curious hereafter to 

 compare them with more careful measurements. 



Before we proceed, however, to the details of the experi- 

 ments, it may be well to give the formulae in a state fitted for 

 immediate application. The light incident on the metal 

 being polarized in a certain plane, let a denote the azimuth 

 of this plane, or the angle which it makes with the plane of 

 incidence; and as the reflected light will be elliptically pola- 

 rized, or,in other words, will perform its vibrations in ellipses 

 all similar and equal to each other, as well as similai'ly placed, 

 put 6 for the angle which either axis of any one of these 

 ellipses makes with the plane of incidence, and let j3 be another 

 angle, such that its tangent may represent the ratio of one 

 axis of the ellipse to the other. Then when the optical con- 

 stants M and X (of which I suppose the first to be a number 

 greater than unity, and the second an angle less than 90°) 

 are known for the particular metal, the angles and j3 may 

 be computed for any value of a, at any given angle of inci- 

 dence, by the following formulae : 



