210 
The Photophone . 
[April, 
Obviously, therefore, if we could devise an apparatus by 
which the sound waves of the voice could undulate a beam 
of light in sympathy with themselves, and project this beam 
to a distant place where it could be received on a piece of 
selenium through which a current flowed on its way through 
a telephone, we should be able to reproduce the original 
voice in the receiving telephone. As early as the summer of 
1878 this idea occurred to one “ L. F. W.,” for in a letter to 
“ Nature” dated Kew, June 3, he describes an arrangement of 
this kind. Such hints, moreover, interesting in an historical 
sense, are, however, of very little account in science unless 
they are followed up by experiment and practically tested. 
“ L. F. W.” appears to have contented himself with throw- 
ing out the suggestion, and to Professor Hell belongs the 
honour of inventing the photophone by dint of patient labour. 
At the very beginning of his attempt he encountered 
serious difficulties. The intractable nature of selenium 
baffled all his efforts. This ambiguous material, which, 
like phosphorus and sulphur, is neither metal nor non-metal, 
was accidentally discovered by Berzelius, the great Swedish 
chemist, when lie was groping for something else — tellurium 
— and the foundling has proved to be the more important 
substance of the two, for owing to its singular property of 
electric sensibility to light it has been chosen from among its 
humbler brethren and lifted into honour. The property in 
question was accidentally discovered by Mr. May, an as- 
sistant of Mr. Willoughby Smith, the electrical engineer, 
who had set Mr. May to measure the resistance of a piece of 
crystalline selenium with a view to employing it in testing 
submarine cables. Much to his surprise Mr. May found the 
resistance of the selenium vary in a strange manner when 
the light fell on it, and fortunately Mr. Willoughby Smith 
took the matter up and verified the effect, and then published 
it to the scientific world. 
The striking effedt was eagerly investigated by a number 
of scientists, who all agreed in referring it to the action of 
light, and the yellowish-green rays of the spectrum were 
found by Professor W. Grylls Adams to be the most potent 
to produce it. Mr. Robert Sabine demonstrated that there 
was a real diminution of the internal resistance of the 
selenium under the influence of light ; but Professor Adams 
also showed that the observed increase of a current flowing 
through the substance was not entirely due to its loss of re- 
sistance but to the adtual generation of a current in the 
selenium. This fadt is a very important one, and will, 
perhaps, find its use hereafter in the transmission of optical 
