354 



NATURE 



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M' 



PHOTOPHONE EXPERIMENTS 



fR. ANDREW ;AMIES0N, C.E., Principal of the 

 Glasgow Mechanics Institution, sends us an account of 

 the following experiments on the photophone, shown by him at 

 1 lecture delivered by him on January 19, before the Glasgow 

 Philosophical Society, on the history of selenium. I 



The effects of light and heat on the conductivity of selenium | 

 were shown by means of a simple and inexpensive form of 1 

 •'cell" joined up in a Wheatstone's Bridge with a reflecting 

 galvanometer. The cell is constructed in the folloHing way : — j 

 A piece of plate-glass or of a glass tube of about an inch diameter 

 and about three inches long is chosen, and upon its exterior are 

 tightly wound two sejiarate parallel wires of No. 25 B.W.G., 

 the wires them?elves being of copi_er covered with silk or cotton. 

 A red-hot iron or poker is then applied to the middle region of 

 the coil of wire so as to burn off the insulating covering of silk 



9. To-and-fro movement of telephone disk. 



10. Vibration of air o| posite the same. 



11. Vil,ration of drum of ear of listener at the telephone and a 

 sound heard. 



Not only the pitch but the tone of the voice was distinctly 

 heard . 



or cotton. The bare wires are cleaned, and the blank cell being 

 raised to the proper temperature, vitreous selenium is rubbed on 

 the wires so as to fill the narrow interspaces left by the removal 

 of the silk covering. The selenium is afterwards annealed in the 

 the usual fashion to render it more highly conductive. One of 

 the cells thus used had resistances of 5740 and 3440 ohms 

 respectively in the dark and in the light ; but others have less 

 re^istances, one being as small as 500 ohms in the dark. The 

 first-named cell (a flat one) was twenty-one days old, and had 

 increased several thousand ohms in that time. 



The musical note of a "singing flame " was reproduced in the 

 telephone by means of one of the annular cells thus formed upon 

 a glass tube in the following manner, suggested by Prof. Blyth 

 (Fig. I) : — The cell, c, joined in circuit with a battery, n, and 

 telephone, T, was placed outside entirely surrounding tht gla'.s 

 tube in which a small gas-jet was "singing." .Speech was after- 

 wards reproduced by the arrangement shown in Fig. 2. At the 



back conical mouthpiece which receives the voice is fixed a mem- 

 brane of goldbeater's skin which forms the front of a chamber 

 A, into which gas is led, and from which a short tube leads to a' 

 small gas-jet, in the manner devised by Kbnig. Opposite the 

 gas-flame was placed the selenium cell in circuit with a battery 

 of twenty cells and a distant telephone. There were thus eleven 

 changes going on simultaneously : — 



1. Muscular movement of speaker's vocal organs. 



2. Vibration of air opposite speaker's mouth. 



3. Corresponding vibrations of the thin membrane. 



4. Variations of pressure controlling the supply of gas to jet. 



5. Hence increase and decrease of gas-flame. 



6. Increase and decrease of resistance of the selenium cell. 



7. Rise and fall of battery current. 



8. Increise and decrease of magnetism in magnet of telephone. 



THE COFFEE-LEAF DISEASE 



'V" WO interesting papers on this subject were read at the last 

 meeting (3rd inst.) of the Linnean Society, the one treating 

 of Its ravages in India, the other its nature and spread in South 

 America. 



In the fir>t Mr. Wm. Bidie, in a letter to Mr. J. Cameron of 

 Bangalore, refers to the Coorg country, situated in the Western 

 Ghats, where European enterprise in coffee has been wholly 

 developed within the l.nst twenty-five years, and no di- ease was 

 observed till four or five years ago. The author mentions 

 that the disease appears to have been imported from Ceylon by 

 way of Chickmoorluor, a district of Mysore, sixty miles distant 

 from Coorg. It seems worst in inpoveri-hed, exposed fields, 

 and lea'-t where there is .shade and rich soil. A small red insect 

 has been noticed feeding over leaves covered with the pest, but 

 what the insect's relation is to the disease as yet remains unde- 

 termined. Hants grown from Ceylon .=eed snffer most, while 

 those trees of Coorg origin and growth are least affected. A 

 system of "renovation-pitting" hns been successfully tried, a 

 pit being dug at short intervals wherein, after judicious pruning, 

 all the aflVcted leaves are buried, and this precaution seems to 

 check the >pread of the disease, particularly among the Coorg 

 coffee-trees. 



In the second communication Dr. M. C. Cooke describes and 

 summarises all the data extant up to the present time of the pro- 

 gressof coffeediseasein South America. Plantations in Venezuela, 

 Costa Rica, Bogota, Caracas, and Jamaica have been affected. He 

 discourses on the nature of the blight, and is of opinion that the 

 disease is a complicated one, being himself as yet unprepared to 

 affirm that either the Septoria, the Spharella, or the Stilbum, 

 three so-called different kinds of fungi, or altogether, is the true 

 cause of the disease. At the same time he thinVs it possible 

 that none of these forms of funjus are autonomous, and that 

 all may be related to each other as forms or conditions of the 

 same fungus, of which Sp/uT'ella is the highest and most perfect 

 manifestation. He observes tl at the discoloured >pots maybe 

 without any visible fungus upon them, and exhibit no trace of 

 mycelium in the tissues, or they may nourish a Septoria, as seen 

 by the Rev. M. J. Berkeley, or a Splurrella as found by himself, 

 or finally a species of Stilbum as seen by Prof. Saenx and him- 

 self Further, the Stilbum may occur on the same spot as the 

 perithecia of the Spluriclla, or both perithecia and Stilbum ; the 

 one without the other may be found occupying different spots. 

 Mr. Cooke admits that altogether it is difficult" satisfactorily to 

 answer the que-tion. What is the cause of this form of coffee 

 disease ? 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 

 Oxford. — Sir William Harcourt announced on Monday that 

 the evidence taken before the Oxford University Cf.mmissioners 

 w ould be laid before Parliament without delay. 



Cambridge. — The first Smith prize has been adjudged to A. 

 R. Forsyth, of Trinity College, Senior Wrangler. K. S. Heath, 

 of Trinity College, Second Wrangler, and A. E. Steinthal, also 

 of Trinity College, Third Wrangler, were equal in the competi- 

 tion for the second prize. 



Mr. W. J. Lewis, M.A., of Trinity College, Cambridge, 

 and Fellow of Oriel College, Oxford, has been elected to the 

 Chair of Mineralogy, in the place of the late Prof. Miller, 

 F. R.S. The University is to be congratulated on having secured 

 as Professor of Mineralogy one so competent to take Prof. 

 Miller's place. 



Mr. A. Scott is giving demonstrations in Elementary Organic 

 Chemistry at the University Laboratory. Mr. J. F. Walker is 

 lecturing on the same subject at Sidney Sussex College. 



Lord Kayleigh is giving a short ciaurse on the Unit of Elec- 

 trical Resistance, and on February 2i will commence an 

 advanced course of lectures on Sound. Mr. Glazebrook is giving 

 demonstrations on Advanced Electricity and Magnetism, and 



