of Refraction of various Substances for the Electric Ray. 297 



semi-cylinder near the radiator, was placed along a diameter of the 

 spectrometer circle. The second semi-cylinder was separated from 

 the first by an air-space 2 cm. in breadth. The plane surfaces of the 

 two semi-cylinders were thus separated by a parallel air-space ; the 

 first semi-cylinder rendered the beam parallel, and the second 

 focussed the rays on the receiver placed opposite the radiator. 

 With the radiator used, I found a thickness of 2 cm. of air-space to 

 be more than sufficient for total reflection of the incident ray.* 



On rotating the cylinders to the right and to the left, two posi- 

 tions for total reflection were obtained. The difference of circle 

 readings for these positions, equal to twice the critical angle, was 

 found to be 58°. The critical angle for glass is therefore 29°. 



^ = 2 04 (2). 



Having thus obtained the value of the index, I tried to find 

 whether it would be possible to obtain approximately good results by 

 measuring the deviation of the refracted ray. In the first series of 

 experiments, I used for this purpose a semi-cylinder, with the 

 radiator at its principal focus (the cylindrical surface being next to 

 the radiator), so that the emergent rays were parallel. On trying to 

 find the angle of refraction corresponding to a given angle of inci- 

 dence, I could obtain no definite reading, as the receiver continued 

 to respond, when moved through five or six degrees on either side of 

 the mean position where the response was strongest. It must be 

 remembered that owing to the finite length of the waves, there is no 

 well-defined geometrical limit to either the ray or the shadow. 

 There is, however, a position for maximum effect, and it is possible 

 with some difficulty so to adjust the sensitiveness of the receiver that 

 it shall only respond to the maximum intensity. 



Another troublesome source of uncertainty is due to the action of 

 the tube which encloses the receiver. When a slanting ray strikes 

 the inner edge of the tube, it is reflected and thrown on to the 

 delicate receiver. Unfortunately it is difficult to find a substance 

 which is as absorbent for electric radiation as lamp-black is for light. 

 Lamp-black in the case of electric radiation produces copious reflec- 

 tion. I have tried layers of metallic filings, powdered graphite, and 

 other substances, but they all fail to produce complete absorption. 

 The only thing which proved tolerably efficient for this purpose was 

 a piece of thick blotting paper or cloth soaked in an electrolyte. A 

 cardboard tube with an inner layer of soaked blotting paper is 

 impervious to electric radiation, and the internal reflection, though 

 not completely removed, is materially reduced. No reliance can, 



* Vide the following paper " On the Influence of the Thickness of Air-space on 

 Total Keflection of Electric "Radiation." 



