1881.] on Seltnumn and its ajqdi cat ions to the Photophone, <£'< 



)33 



successful, that altliougli the pictures liitliorto transmitted arc of a 

 very rudiiiieutary eliantetcr, 1 tliiiik there can be little doubt that 

 furtlicr elaboration of the instruineut would render it far more 

 etleetive. 



Iodide of potassium is very easily decomposed by a current of 

 electricity. If a piece of paper which has been soaked in a solution 

 of this substance be laid on a piece of metal M, Fig. 2, which is con- 

 nected to the negative polo of a battery B, and a piece of platinum 



wire r, which is connected with the positive pole, be drawn over its sur- 

 face, the path of the point will be marked by a brown line, due to the 

 liberation of iodine. Let the jjlatinum wire and the metal j^late bo 

 connected to a second battery B' in such a manner that a current of 

 electricity may pass througli the paper in the opposite direction; and 

 let a variable resistance R be inserted between the platinum wire 

 and the first battery B, and a selenium cell S between the platinum 

 wire and the second battery. And let the resistance be so adjusted 

 that when the selenium cell is exposed to a strong light, the two 

 opposite currents through tlie paper and the galvanometer G 

 neutralise each other ; tlien the point when drawn over the pajier 

 will make no mark. But if the selenium cell is shaded, its resistance 

 will be immediately increased, and the current from the first battery 

 will predominate. The point, if moved over the paper, will now 

 trace a strong line, which, if the selenium is again ex2)0sed, will be 

 broken ofi" or enfeebled according to the intensity of the light. (Exp.) 

 If a sewes of these brown lines were drawn parallel to one another 

 and very.close together, it is evident that by regulating their intensity 

 and introducing gaps in the proper places any design or picture might 

 be represented. This is the principle of Bakewell's copying telegraph, 

 which will transmit writing or pictures drawn upon tinfoil with a 

 non-couducffing ink. My instrument differs from his in that the 

 current is varied simply by the action of light. The transmitting 

 instrument Y, Fig. 3, consists of a small cylindrical box 2 inches 



