592 Prof. Brace on the Change produced by the 



Fizeau used a combination o£ elements to compensate for 

 the dispersion and the unavoidable double refraction and, 

 also, to magnify the expected rotation of the ray. He further 

 readjusted the beam of sunlight, which he used, on reversing 

 the system, and this seems to have given him variations in 

 bis readings. These possible sources of spurious effects have 

 been eliminated in the arrangement which I used. 



His experiment is based on Brewster's law for a refracted 

 ray, namely, 



. „ tan a 



tan /3= T . r, 



cos {i — r) 



where i and r are the angles of incidence and refraction aud 

 a. and /3 the corresponding azimuths, respectively, of the 

 plane-polarized ray. Any change in the index would indi- 

 cate itself by a variation in the angle r and, hence, in the 

 azimuth (3. Fizeau usually used, for producing the effect, 

 two so-called rotary piles, each containing four slightly 

 prismatic plates of glass 1 to 2 millimetres thick, and 

 mounted slightly inclined to one another. In his experiment 

 the incidence of the ray upon these piles was 70°, and the 

 azimuth of the polarized beam, usually 20°, sometimes 30°. 

 He also generally used two piles, at a smaller inclination, for 

 compensating the elliptical polarization due to the double 

 refraction in the rotary plates. For compensating the rotary 

 dispersion, he inserted during a part of his observations a 

 natural rotary substance, such as quartz, or some of the 

 essential oils. Finally, to increase the effect, he inserted 

 several " amplifying " piles which magnified the effect some 

 30 times. His polariscope consisted either of crossed nicols, 

 an interference system, or the sensitive-tint single or biquartz 

 systems. Such a complicated arrangement necessarily caused 

 a great loss of light and thus reduced his sensibility. 



In my preliminary experiments I adopted a similar 

 arrangement to that of Fizeau. In order to avoid as far as 

 possible the disturbing factor of double refraction, ten plates 

 were cut from well-annealed optical glass nj> — T5178. 

 These plates were ground wedge-shaped so that the internally 

 reflected images would not enter the field of view. They 

 were 20 mm. long and 15 mm. wide, and their faces made an 

 angle with one another of approximately 50' in the direction 

 of their greatest lengths, their thickness diminishing from 

 0'6 mm. to 0'3 mm. These plates were set on rings, upon 

 which they could turn, whose planes made an angle of 70° 

 with the vertical plane, in which plane a second ring, carrying 

 the first ring, could be made to rotate. Thus, with an 



