NATURE 



4S1 



THURSDAY, SEPTEMBER 23, iSSo 



THE PHOTOPHONE 



EARLY in the summer of the current year it was 

 announced in the columns of Nature that Prof. 

 Graham Bell had made a discovery which, for scientific 

 interest, would rival the telephone and phonograph, and 

 that he had deposited in a sealed packet, in the custody of 

 the Smithsonian Institute, the first results of his new 

 researches ; that announcement has now received its 

 due fulfilment in the lecture by Prof. Bell to the Ame- 

 rican Association for the Advancement of Science, on 

 Selenium and the Photophone, which will be found 

 on another page. In spite of those who ingenuously 

 attempted at the time of our announcement to fore- 

 stall Prof. Bell and to discredit the idea that he had 

 done anything new, the discovery, which he has now 

 published, is a startling novelty. The problem which 

 he has attacked is that of the transmission of speech, 

 not by wires, electricity, or any mechanical medium, 

 but by the agency of light. The instrument which em- 

 bodies the solution of this principle he has named the 

 Photophone. It bears the same relation to the telephone 

 as the heliograph bears to the telegraph. You speak to 

 a transmitting instrument, which flashes the vibrations 

 along a beam of light to a distant station, where a 

 receiving instrument reconverts the light into audible 

 speech. As in the case of that exquisite instrument, the 

 telephone, so in the case of the photophone, the 

 means to accomplish this end are of the most ridi- 

 culous simplicity. The transmitter consists of a plane 

 silvered mirror of thin glass or mica. Against the 

 back of this flexible mirror the speaker's voice is 

 directed ; a powerful beam of light is caught by a lens 

 from the sun and directed upon the mirror, so as to be 

 reflected straight to the distant station. This beam of 

 light is thrown by the speaker's voice into corresponding 

 vibrations. At the distant station the beam is received 

 by another mirror, and concentrated upon a simple disk 

 of hard rubber fixed as a diaphragm across the end of a 

 hearing-tube. The intermittent rays throw the disk into 

 vibration in a way not yet explained, yet with sufficient 

 power to produce an audible result, thus reproducing the 

 very tones of the speaker. Other receivers may be used, 

 in which the variation in electrical resistance of selenium 

 under varying illumination is the essential principle. 

 The experimental details have been worked out by 

 Prof. Bell in conjunction with Mr. Sumner Tainter. 

 They have discovered that other substances beside 

 hard rubber, gold, selenium, silver, iron, paper, and 

 notably antimony, are similarly sensitive to light. This 

 singular production of mechanical vibrations by rays 

 of light is even more mysterious than the produc- 

 tion of vibrations in iron and steel by changes of 

 magnetisation. It was indeed this latter fact which led 

 the discoverers to suspect the analogous phenomenon of 

 photophonic sensibility in selenium and in other sub- 

 stances. Hitherto, in consequence of the mere optical 

 difficulties of managing the beam of light, the distance 

 to which sounds have been actually transmitted by the 

 Photophone is less than a quarter of a mile, but there is 

 Vol. XXII.— No. 569 



no reason to doubt that the method can be applied to much 

 greater distances, and that sounds can be transmitted 

 from one station to another wherever a beam of light can 

 be flashed ; hence we may expect the slow spelling out of 

 words in the flashing signals of the heliograph to be 

 superseded by the more e.xpeditious whispers of the 

 Photophone. 



We congratulate Prof. Bell most sincerely on this 

 addition to his well-won laurels, and venture to predict 

 for his photophone a great, if not a widely-extended, 

 future of usefulness. Silvanus P. Thompson 



THE GEOLOGY OF LONDON 

 Guick to the Geology of London and the Neighbourhood. 

 (An Explanation of the Geological Survey Map of 

 " London and its Environs," and of the Geological 

 Model of London, in the Museutn of Practical Geology.) 

 By William Whitaker, B.A., F.G.S. Third Edition. 

 (London : Longmans and Co. and Edward Stanford, 

 1880.) 



SINCE the appearance of the second edition of this 

 most useful little work, some valuable additions have 

 been made to our knowledge of the rocks which underlie 

 the London Basin. The deep borings at Turnford, near 

 Chcshunt, and at Ware, which were executed by the New 

 River Company in 1879, have furnished new data to 

 geologists for determining the position and characters of 

 that great underground ridge of palaeozoic rocks, the 

 probable existence of which was so long ago indicated by 

 Mr. Godwin-Austen. This underground ridge of palasozoic 

 rocks has now been reached in no less than six borings, 

 those of Kentish Town, Harwich, Crossness, Meu.x's 

 Brewery, Turnford, and Ware. In four of these cases 

 the age of the rocks which have been found unconform- 

 ably underlying the Cretaceous strata is placed beyond 

 question, by the discovery of the well-preserved and 

 characteristic fossils, lists of which have recently been 

 published by Mr. Etheridge. At Harwich the cores of 

 dark-coloured indurated shale yielded Fosidonice, which 

 proved that the rock belongs to the lowest part of the 

 Carboniferous system; at Meux's Brewery and at Turn- 

 ford the purple shales yielded the characteristic fossils 

 of the Upper Devonian ; while at Ware cores were 

 brought up crowded with the well-known fossils of the 

 Wenlock shale. 



But in the case of the Kentish-Town and Crossness 

 borings no fossils have been detected in the cores of rock 

 brought up from the infra-cretaceous rocks, and in these 

 cases geologists have had to fall back upon the far less 

 satisfactory evidence afforded by their mineral characters. 

 Under these circumstances there has arisen considerable 

 diversity of opinion as to the age of red, mottled, and 

 variegated strata which have been reached in these two 

 borings. While Prof. Prestwich is inclined, to refer the 

 rocks in both cases to the Old Red Sandstone, the officers 

 of the Geological Survey are in favour of regarding them 

 as belonging to some part of the New Red Sandstone or 

 Poikilitic. Mr. Whitaker has so well stated the objec- 

 tions to the view that the rocks at Kentish Town and 

 Crossness are of Old-Red-Sandstone age, that we cannot 

 do better than quote what he says upon the subject : — 



" 1 may here remark that there is a strong reason 



