A^ov. 4, 1880J 



NATURE 



19 



circuit without passing somewhere through selenium 

 from one brass disk to the next. The special advantages 

 of the " cell " devised by Prof. Bell are that in the first 

 place the thickness of the selenium that the current 

 must traverse is nowhere very great ; that in the second, 

 this photo-electrical action of light on selenium being 

 almost entirely a surface action, the arrangement by 

 which all the selenium used is a thin surface film could 

 hardly be improved upon ; and that thirdly, the symmetry 

 of the cylindrical cell specially adapts it for use in the 

 parabolic mirror. Those details will be of great interest 

 especially to those who desire to repeat for themselves the 

 experimental transmission of sound by light. The greatest 

 distance to which articulate speech has yet been trans- 

 mitted by the selenium-cell-photophone is 213 metres, or 

 233 yards. 



When sunlight is not available recourse must be had 

 to an artificial source of sufficient power. During the 

 recent experiments made by Prof. Bell in Paris the 

 weather has been adverse, and the electric light has been 

 called into requisition in the ateliers of M. Bregnet 

 (Fig. 7, which is kindly supplied us by Prof Bell). The 

 distance in these experiments between the transmitting 

 diaphragm B and the parabolic reflector c c of the receiver 

 was fifteen metres, the entire length of the room in which 



7.— The Photophone with Electric Light. 



the experiments were made. Since at this distance the 

 spoken words were themselves perfectly audible across 

 the air, the telephones connected with the selenium-cell 

 were placed in another apartment, where the voices were 

 heard without difficulty and without doubt as to the 

 means of transmission. 



Of the earlier and less perfect forms of the photophone 

 little need be said. One device, which in Prof Bell's 

 hands worked very successfully over a distance of eighty- 

 six yards, consisted in letting the beam of light pass 

 through a double grating of parallel slits lying close to 

 one another, one of which was fixed, the other movable 

 and attached to a vibrating diaphragm. When these 

 were placed exactly one in front of the other the light 

 could traverse the apparatus, but as the movable grating 

 slid more or less in front of the fixed one more or less of 

 the light was cut off. Speaking to the diaphragm there- 

 fore caused vibrations which shut or opened, as it were, 

 a door for the beam of light, and altered its intensity. 

 The mirror transmitter of thin glass silvered was however 

 found superior to all others ; and it is hard to see how it 

 could be improved upon, unless possibly by the use of a 

 thin disk of silver itself accurately surfaced and polished. 



Whatever be the future before the photophone, it 

 assuredly deserves to rank in estimation beside the now 

 familiar names of the telephone and the phonograph. 



NOTES . 



The Triennial Gold Medal of the Chemical Section of the Philo- 

 sophical Society of Glasgow, founded in commemoration of the 

 work of Thomas Graham, F.R.S., late Master of the Mint, will 

 be awarded, at the end of the present session, for the best paper 

 on any subject in pure or applied chemistry. Authors are 

 requested to send in their papers not later than February I, 18S1, 

 addressed to the Secretary of the Section, Ur. J. J. Dohbie, 

 Chemical Laboratory, Univftsity of Glasgow. 



_ Th E annual meetuig of the five academies which constitute 

 the French Institute was held on Monday last week, when M. 

 E. Levasseur gave an address on the Ethnography of France, 

 and Col. Pcrrier described the operation he undertook to connect 

 geodetically Algeria and Spain. 



The Royal Institution Lecture arrangements (not yet complete) 

 for the ensuing season (before Easter) will include the Christmas 

 course by Prof. Dewar ; and courses by Professors Tyndall and 

 Schafer, the Rev. William Ilaughton, the Rev. H. R. Havveis, 

 Mr. H. H. Statham, Mr. Reginald S. Poole, and others. 

 Friday Evening Discourses will probably be given by Mr. 

 Warren De La Rue, Prof. Tyndall, Sir John Lubbock, Sir 

 William Thomson, Dr. J. Burdon Sanderson, Dr. Andrew 

 Wilson, Dr. Arthur Schuster, Mr. Alexander Buchan, Dr. 

 W. H. Stone, Dr. W. J. Russell. 



The death is announced of Sir Thomas Bouch, the engineer 

 of the Tay Bridge. It is believed that his system received a 

 severe shock on account of the Tay Bridge disaster and the 

 proceedings consequent on it. 



M. Erhard, the well-known French cartographer, died on 

 October 23. M. Erhard was a naturalised Frenchman, having 

 been born at Freiburg-im-Breisgau. 



Among Mr. Stanford's announcements of forthcoming works 

 are the following : — " Prehistoric Europe: a Geological Sketch," 

 by Dr. James Geikie, F.R.S. ; a fourth edition of " The Coal- 

 Fields of Great Britain," by Prof. Edward Hull, F.R.S. ; 

 "Life and her Children: Glimpses of Animal Life from the 

 Amccba to the Insects," by Arabella B. Buckley ; "Index Geo- 

 graphicus Indicus : a Gazetteer of India," by J. F. Barness ; 

 "The Flora of Algeria, considered in Relation to the Physical 

 History of the Mediterranean Region and Supposed Submergence 

 of the Sahara," by W. Mathews ; " Water Supply of England 

 and Wales : its Geology, Underground Circulation, Surface 

 Distribution, and Statistics," by C. E. de Ranee. 



In the November number of Scribner's Monthly is a curious 

 article on Second Sight or Clairvoyance, by an " Ex-Conjuror" 

 (Mr. Henry Hatton), in which it is shown that the whole thing 

 is an elaborate system of mnemonics. The article has all the 

 appearance of being genuine. 



In reference to the notice in Nature, vol. xxii. p. 587, on 

 the address of Dr. Karl Zittel on the subject of the geology of 

 the Libyan Desert, we should state tliat while the paper contains 

 Zittel's opinions of the observations and collections of other 

 travellers, it is mainly derived from the Professor's personal 

 examination of the physiogr.iphy of that country, and of the 

 fossils which he there collected, when, as a member of the expe- 

 dition under the leadership of Dr. Rohlfs, he vi-ited the Libyan 

 Desert in the winter of 1873-74. 



The lecture on "The Modifications of the External Aspects 

 of Organic Nature produced by Man's Interference," delivered 

 by Prof. RoUeston to the Royal Geographical Society in 1879, 

 has just been published in that Society's Journal. Amongst 

 other interesting matters Prof. RoUeston rectifies an error into 

 which all or most translators of Cresar ;have fallen respecting 



