Prof. S. P. Thompson on Polarizing Pri 



nsms. 435 



instrument; in it we have the dial in a cylindrical shape. This 



cylindrical dial is mounted on a speed-indicator N; and the 



Ffo. 1. Fi S- 2 - 



— tension -> 



Lk 



M 



^velocity-^ 



§ M 



pointer M is controlled by the tension spring-rod P Q; and 

 thus M points directly to the figures indicating the rate 

 at which energy, in foot-pounds per second, is being trans- 

 mitted. In order that the speed-indicators may be calibrated 

 with great accuracy, they are attached to wheelwork driven by 

 a water-wheel of large power in comparison with that required 

 to drive the indicator ; if the water-wheel be well balanced 

 and fed by a constant head, very good results can be obtained. 

 The velocity of the water-wheel is measured by means of a 

 counter read after short periods of time. 

 Taunton, May 18, 1883. 



LXVI. On Polarizing Prisms. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the very able paper of Mr. P. T. Glazebrook, F.R.S., 

 which appears in your May issue, a polarizing prism is 

 described which, according to the description, " differs from 

 the one described by Professor S. P. Thompson (Phil. Mag. 

 Nov. 1881) only in the fact that its ends are normal to its 

 length instead of being inclined obliquely to it." 



I was not aware that in my paper " On a New Polarizing 

 Prism," which you did me the honour to print in the Philo- 

 sophical Magazine for November 1881, I had anywhere 

 limited myself to any one special angle for terminal faces to 

 the prism. I did discuss in general terms what the result 



