482 Dr. E. Bornstein on the Influence of Light 



really due to light, and not to a rise of temperature, which 

 always accompanies any exposure to radiation. The metals 

 that have been examined differ from selenium and tellurium 

 in this respect, that their resistance invariably increases with 

 the temperature ; and when they are exposed to light, the ob- 

 served change in their conductivity arises from the differential 

 action of two causes, namely the direct action of light and 

 the rise of temperature ; and the existence of the first of these 

 may be considered as proved beyond question, if the observed 

 change consists in a diminution of the resistance. 



Now, in the case of platinum, gold, and silver, phenomena 

 have been observed which are exactly similar to those already 

 noticed in the case of selenium and tellurium ; and their detec- 

 tion was effected by giving these metals such a form that the 

 surface was very large as compared with the mass, so that as 

 much as possible of the mass was exposed to the incident 

 luminous rays. For the exposure a collimating tube with a 

 slit was employed, a lens like that in use in spectroscopic 

 experiments, and a flint-glass prism of 45° refracting angle, 

 placed in the position of minimum deviation for sodium- 

 light. The achromatic collimating lens had a focal length of 

 260 millims., and a diameter of 26 millims. In front of the 

 slit was a Bunsen burner, the flame of which was coloured in 

 the usual manner by means of a bead of salt on a platinum 

 wire. When an observation was to be taken without expo- 

 sure to light, the flame was extinguished, and the slit was 

 covered with a piece of black paper. In a few cases sunlight 

 was used — sometimes the direct rays, and on other occasions 

 the ray after its refraction through a prism being employed. 

 The actual observations consisted in comparing the measured 

 resistances of the metallic bodies when unexposed, with their 

 resistances when exposed to light. 



In order to avoid any risk of error, two different methods 

 of measuring the resistance were employed — the measurement 

 by the Wheatstone bridge arrangement, and the measurement 

 of a resistance according to Weber's method of " damped vibra- 

 tions." Among the experiments a few were also made with 

 induced currents ; and though they are inconclusive as re- 

 gards the action of light, they are put forward with the rest 

 because they enable us to draw some interesting inferences 

 concerning the action of a current upon its conductor. In 

 employing the first method, two equal metallic bodies were 

 simultaneously observed while one and then the other was 

 alternately illuminated; and the ratio of their resistances, 

 under these circumstances, was determined each time inde- 

 pendently. This method of procedure had this advantage, 



