THE PHOTOPHONE. 155 



and then his apparatus being steadily set up, he removes the covering from 

 the lens. To develop the image (in the dusk of the evening or by candle- 

 light) it is necessary to put some drops of ammonia in a saucer, breathe 

 upon the plate so as to soften the collodion, and hold it above the ammonia, 

 and then, under the influence of the vapour, the picture will appear. After 

 this simple operation the picture will be found fixed for a lengthened period 

 practically indefinitely. Thus on the return of the pedestrian he can 

 reproduce, at small expense, a whole series of little pictures faithfully repre- 

 senting his holiday tour. The illustration shows a small apparatus by 

 which on thin plates small photographs can be taken and fixed till it is 

 found desirable to enlarge them. 



The Photophone^ one of the most recent contributions to science, is 

 an instrument which, in combination with the telephone principle, makes it 

 possible to convey sounds by means of a ray of light, and by means of a 

 '- quivering beam " to produce articulate speech at a distance. The success 

 of the Photophone depends upon a rare element, selenium, which has its 

 " electrical resistance " affected by light. Professor Adams demonstrated 

 that the resistance of silenium was reduced just in proportion as the 

 intensity of the light which was acting upon it. Here was the key to the 

 Photophone as thought out by Professor Bell. He fancied that he might 

 by means of his telephone produce sound if he could vary the intensity of 

 the beam of light upon the selenium, which he connected with his telephone 

 and battery. 



The Photophone consists of a transmitter for receiving the voice and 

 conveying it along the beam of light, and a receiver for taking the light 

 and converting it into sound the receiver being the telephone. There is 

 a small mirror (silvered mica has been used) suspended freely for vibration. 

 A lens is used to transmit to this the beam of light, and this beam is again 

 reflected by another lens to the receiver, which consists of a reflector which 

 has a cell of selenium in its focus, connected, as already stated, with the 

 telephone and battery. The speaker stands behind the mirror, and the sound 

 of his voice against the reverse side makes it vibrate in unison with the sounds 

 uttered. The movements cause a quivering in the reflected beam, and this 

 in its changing intensity acts on the selenium, which changes its resistance 

 accordingly, and through the telephone gives forth a sound ! 



This is the apparently complicated but really simple, and at the same 

 time wonderful, invention of Professor Bell. By the Photophone not only 

 sounds but movements can be converted into sound ; even the burning of 

 a candle can be heard ! The Photophone is still capable of improvement, 

 and has not as yet arrived at its full development, for it is stated it can be 

 made quite independent of a battery or telephone. 



There are many phenomena connected with the Polarization of Light. 

 This requires some notice at our hands. We know that a ray of ordinary 

 light is supposed to be caused by vibrations of the highly attenuated medium, 

 aether. These vibrations occur across the direction of the ray; but when 



