Action of Selenium Cells. 219 



In the beginning of April * I repeated the experiment of 

 Mr. Graham Bell, and allowed light to pass through ebonite. 

 In order to be able to make a quantitative observation I con- 

 nected the selenium cell ; on which the light was to fall, not 

 only with the telephone, but also with a galvanometer. 

 While, however, it appeared that a photophonic effect took 

 place through the ebonite, it was shown that this effect was 

 only a small part of the direct effect when the ebonite dia- 

 phragm was away. Now, as ebonite allows passage only 

 to red and ultra-red rays, we learn from this experiment two 

 things : — 



^ (1) That the illuminating rays are those which are especi- 

 ally absorbed by selenium, and that these produce the greater 

 part of the photophonic action. 



(2) That it is even possible to make a selenium photophone 

 without light — that is, with exclusion of illuminating rays and 

 by the influence of heat-rays only. 



That there is simply a heating effect of the illuminating 

 rays in the selenium photophone has perhaps not yet been 

 sufficiently insisted on, because the most modern researches 

 on light-rays are not yet assimilated to the general view of 

 physics. We find still the conception of three different kinds 

 of rays — heating, illuminating, and actinic: whilst it has long 

 been demonstrated that there is only one form of rays, differing 

 from one another, however, in wave-length and intensity. 

 On the body on which the ray falls depends whether its 

 energy is perceived as heat, or light, or chemical effect. In 

 order to be effective a ray must be absorbed. The bodies, 

 however, on which the rays fall select the rays they absorb in 

 the most various manner, which we recognize by the endless 

 varieties of absorption-spectra. The absorbed rays alone are 

 able to exert an effect ; they only can warm the body. And 

 heating occurs not only by red and ultra-red rays, but by the 

 rays which are absorbed. Only the absorbed rays can pro- 

 duce (and that is the point in question here) changes of 

 volume and of shape, and in this way influence the contact 

 of current-conducting parts. 



As the last-mentioned experiment demonstrates, selenium 

 absorbs principally the illuminating rays. When, therefore, 

 selenium is exposed to radiation, the change of volume and of 

 shape is produced chiefly by the illuminating rays ; selenium 

 is heated by light. It must be the illuminating rays which 

 make the selenium cell act microphonically. yL 



That light, however, may produce in selenium other 

 changes than heat and deformation, which are essential to 

 * Lond. Phys. Soc. April 9, 1881 ; ' Nature,' xxiii. p. 595. 



