1897.] 



on the Polarisation of the Electric Bay. 



301 



from the optical index, and the focal distance of a glass lens for light 

 gives no clue to its focal distance for electric radiation. I found, for 

 example, the index of refraction of a specimen of glass to be 2 • 04, 

 whereas the index of the same specimen for sodium light is only 

 1-53. 



There are again many substances, like the various rocks, wood, 

 coal-tar, and others, whose indices cannot be determined owing to 

 their opacity to light. These substances are, however, transparent 

 to electric radiation, and it is therefore possible to determine their 

 electric indices. For the determination of the index, the prism- 

 method is not very suitable. I found the following method, of which 

 I shall exhibit the optical counterpart, to yield good results. When 

 light passes from a dense to a light medium, then, at a certain critical 

 angle, the light is totally reflected, and from the critical angle the 

 index can be determined. I have here a cylindrical trough filled 

 with water. Two glass plates inclosing a parallel air-film are sus- 

 pended vertically across the diameter of the cylinder, dividing the 

 cylinder into two halves. The cylinder, mounted on a graduated 



Fig. 3. 

 (The dotted lines show the two positions of the air-film for total reflection.) 



circle, is adjusted in front of an illuminated slit, an image of the slit 

 being cast by the water-cylinder on the screen. The divergent beam 

 from the slit, rendered nearly parallel by the first half of the cylinder, 

 is incident on the air-film, and is then focussed by the second half of 

 the cylinder. As the cylinder is slowly rotated, the angle of incidence 

 at the air-film is gradually increased, but the image on the screen 

 remains fixed. On continuing the rotation you observe the almost 

 sudden extinction of the image. I say almost, because the light is not 

 monochromatic, and the difierent components of white light undergo 

 total reflection in succession. Just before total extinction the image 

 you observe is reddish in colour, the violet and the blue lights being 

 already reflected. On continuing the rotation the image is completely 

 extinguished. Kotation of the cylinder in an opposite direction gives 

 another reading for total reflection, and the diff'erence of the two 

 readings is evidently equal to twice the critical angle. 



In a similar way I have been able to determine the indices of re- 

 fraction of various substances, both solid and liquid, for electric 

 radiation. In the case of solids, two semi-cylinders, separated by a 



