.s/A' WILLIAM SIEMENS, F.R.S. 411 



his researches, which were communicated to the Berlin Academy, 

 Irtl him, was that the resistance of selenium, and probably, indeed, 

 of all substances, varied inversely to the amount of heat which 

 they contained ; and the reason why selenium showed such extra- 

 ordinary changes under the influence of light was, that under that 

 influence, it changed from one aggregate condition to another 

 from an amorphous to a crystalline condition ; and that at the 

 moment when this change took place, a great deal of heat was 

 absorbed, and therefore the specific heat of the selenium was very 

 much increased. This was strictly a molecular change, and bore 

 on the further discovery which Professor Graham Bell had made, 

 that he could hear the changes going on even in gaseous bodies, 

 produced by the passage of light. The little instrument which he 

 {Dr. Siemens) had constructed to show the members of the Royal 

 Institution was on the table. It had the form of an eye, and on 

 opening the lids, a lens was presented to the light ; through that 

 lens, the light, falling upon it, was concentrated upon a spot in 

 the interior of the ball. At that spot one of the selenium gratings, 

 which had been described, was placed, a grating not larger than a 

 threepenny piece, consisting of five wires laid in zigzag fashion ; 

 one wire was connected to the positive, and the other to the nega- 

 tive pole of a battery. These wires, lying close together, but not 

 touching, were laid on a plate of mica ; a drop of selenium was 

 placed upon them, and this small quantity sufficed to produce the 

 desired results. The principal object he had in devising it was to 

 construct a selenium photometer ; but a difficulty arose in using 

 it for that purpose, because selenium got fatigued under the influ- 

 ence of light. The eye, after being exposed for any considerable 

 period to an intense light, became insensitive, and the lids had to 

 be closed ; it had to go to sleep for some time before it regained its 

 sensitiveness, and the analogy to the human eye went even further 

 than that. If the eye were used after having been kept in the dark 

 for a length of time, it would detect the slightest gleam of light, 

 and mark it on the galvanometer, whereas after it hud been once 

 used in intenser lights, a small gleam would be utterly lost upon it, 

 until it had again had ample rest. The instrument before them 

 had not been used for some years, and it might still be active, but 

 .the audience would have to take the Chairman's word for it, since 

 the galvanometer in circuit with the " eye " was not one whose 



