468 On the Transmission of .Radiation through Ebonite, 



of absorption of a plate of ebonite -^ of an inch in thickness 

 = 1*8 ; and a calculation will show that any rays which can 

 penetrate through J of an inch of ebonite will only have an 

 intensity of — -i-— — that of the resultant beam, without de- 



^ 1650 00 t . to • 



ductino- any thing for the scattering of light. In fact, with the 

 electric light and a wide slit an hour's exposure produced no 

 effect on the photographic plate when'ebonite J in. in thickness 

 was placed before the slit. It must, however, be remembered 

 that ebonite varies in quality ; sometimes the outside alone is 

 black, the inner portions resembling gutta percha in colour. 

 With specimens of this sort a greater thickness could no doubt 

 be traversed than -J inch. In such a case, however, we doubt 

 if the substance would be true ebonite. 



In a communication to ' Nature/ Messrs. Ayrton and Perry 

 show how they determine the refractive index of ebonite by 

 an arrangement with the telephone. They use a prism ; and 

 we should judge by the figure they give that the thickness 

 of ebonite traversed must be about ^ of an inch ; so that the 

 radiations transmitted must be very small. We may remark 

 that the direction of a beam of light issuing from a prism 

 formed of a turbid medium would not have its maximum in- 

 tensity in the true direction of refraction ; it would be slightly 

 displaced. Mr. Preece, in a recent communication to the 

 Eoval Society, remarked that some ebonite he tried was as 

 transparent as rock-salt ; and so it is if a thin-enough layer 

 be taken ; and we think that it was the minute layer 

 that was taken that caused this expression to be used. He 

 also stated that another sample equally thin was perfectly 

 opaque to radiation. Through his kindness we were able to 

 experiment with the identical samples to which he refers. 

 The " transparent " specimen behaved as that we have already 

 described ; the opaque one showed that the radiations were 

 more scattered in their passage through it. We may state 

 that, by examining the thin ebonite with which we first expe- 

 rimented, we could see a trace of the sun's image through the 

 material, and very faintly through two layers. The radiations 

 of low refrangibility were evidently more copiously passed, 

 since when an image of the sun formed by a lens was caused 

 to fall on a piece of paper and a sheet of thin ebonite inter- 

 posed, if the eye or hand was placed at the focus considerable 

 warmth was felt. 



It became interesting to know whether the ebonite was 

 merely a mechanical mixture of sulphur and india rubber or a 

 chemical combination. Placing a piece of stout india rubber, 

 about the same thickness as the ebonite, before the slit of the 

 spectroscope, and with an exposure of ten minutes, no vestige 

 of an image was found on development of the plate. This 



