620 MR. R. T. GLAZEBROOK ON THE REFRACTION OE PLANE LIGHT, &c. 
The comparison is also extended to some earlier experiments of Seebeck’s (Pogg. 
Ann. xxi., xxii., and xxxviii.). As far as one can judge from the details given, the 
differences are hardly greater than the possible errors of the experiment, for in all 
cases lamp light was used and the position of the plane of polarization determined by 
the quenching of one of the rays in a double image prism, and both these circum¬ 
stances preclude great accuracy. On the other hand, the number of individual 
observations was very large, so that the mean deserves considerable weight. 
