178 Mr. S. Bid well on the Sensitiveness 



resolves itself into the plotting of the two curves r — (}>(0), 

 r = ^r(0), between the limits = «i, = a 2 > the fixing of the 

 instrument so that its axis passes through 0, the pole chosen 

 in plotting the curves, the driving of the two pointers Si and 

 S 2 along the two curves (which is easily done by one person) 

 causing S 3 to trace a third curve between the same limits ; 

 then the clamping down of S 1; the forcing of S 3 to follow 

 back its former track, and the reading of the revolutions of 

 the wheel attached at S 2 during this last part of the perform- 

 ance. This reading gives the required value of the integral 

 when the scales on which the two curves were drawn are taken 

 into account. These scales must be chosen so that the maxi- 

 mum difference between OSi and 0S 2 shall at least not be 

 greater than the diameter of the circle whose radius is CSj. 

 In the above description, the word collar is used to mean any 

 form of connection that allows with as little friction as possible 

 one degree of relative freedom. 



Melbourne, June 1, 1885. 



XXII. On the Sensitiveness of Selenium to Light, and the 

 Development of a similar Property in Sulphur. By Shel- 

 FGRD BlDWELL, M.A., LL.B* 



THE remarkable property apparently possessed by crystal- 

 line selenium of having its electrical resistance varied 

 by the action of light, a property which was first announced 

 by Mr. Willoughby Smith in 1873, has been the subject of 

 many investigations!. Of these the best known, and by far 

 the most exhaustive, are the researches of Prof. W. G, Adams 

 and Mr. R. E. Day, an account of which is published in the 

 Phil. Trans, of 1877. As the result of numerous experiments, 

 these gentlemen were led to form the opinion, that "the 

 electrical conductivity of selenium is electrolytic "J. The 

 principal reasons given for this conclusion are:— (1) that the 

 resistance of the selenium-bars used appeared to depend upon 



* Communicated by the Physical Society ; having been read at the 

 Meetings on May 23 and June 13. 



t Willoughby Smith, Journ. Soc. Tel. Eng. ii. p. 31; Earl of Kosse, 

 Phil. Mag. March 1874, p. 161 ; Sale, Proc. Roy. Soc. 1873, p. 283; Phil. 

 Mag. March 1874; Werner Siemens, Phil. Mag. November 1875, p. 416; 

 Draper and Moss, ' Chemical News/ xxxiii. p. 1 ; Adams and Day, Proc. 

 Roy. Soc. 1876, p. 113 ; Phil. Trans. 1877, p. 313 ; C. W. Siemens, Proc. 

 Roy. Inst. 1876, p. 68 ; Sabine, Phil. Mag. June 1878, p. 401 ; Graham 

 Bell, < Nature/ xxii. p. 500 ; Shelford Bidwell, Phil. Mag. April 1881, and 

 January 1883; Fritts, < Electrical Review/ March 7, 1885, p. 208. 



X Phil. Trana. vol. 167, p. 328; Proc. Rov. Soc. 1876, p. 115. 



