50D 



NA TURE 



\_ScpL 23, 1880 



screen, H, which is placed in the principal focus of K. The 

 rays from this image diverge in all directions, and some are 

 taicen up by the lens K and restored to parallelism ; some of 

 these are reflected by the unsilvered mirror, F, down to the field- 

 glass, D, and if this is focussed for parallel lays, as is the case 

 in looking at distant objects, an image of the sun is seen pro- 

 jected on the same field of vie\^' as that of the distant object. 

 x\.s the mirrors E and 7^ are adjusted strictly parallel, the rays 

 proceeding from Fiwio the field-glass are parallel and in the 

 opposite direction to those going from the mirror A to E, which 

 form part of the same pencil as those going to the distant object. 

 Hence the image of the sun seen in the field exactly covers the 

 object to which the sun-flash is visible, and in whatever direction 

 the mirror A is moved so as to alter the direction in which rays 

 are reflected to the distant object, and the angle at which part 

 impinge on E and are reflected through the lens K, the image 

 visible in the glass moves in the same direction. Several 

 attempts to produce this result were made by the use of 

 mirrors and prisms, before the lens K was introduced, but they 

 all failed. It was easy to'make the image of the sun cover the 

 object Avhen the two occupied the centre of the field of view, 

 but directly the mirror \\'as inclined so as to direct the rays not 

 strictly parallel to the axis of the field-glass, the apparent image 

 diverged generally in the same direction along one co-ordinate, 

 and in the opposite along one at right angles to it, so that 

 now here, but in one line across the field, did the image lie in the 

 desired position. The mirrors E and F are adjusted parallel 

 once for all, by noticing the position on a screen of the small 



spot of light reflected from the front of j^as the light passes 

 from E\,Q K. The mirrors are moved by the adjusting screws 

 till this spot has, to the bright reflection from the mirror A, tlie 

 same relative position that the centre of mirror F has to the 

 mirror A, 



In actual use the field-glass is first fixed in position pointing 

 to the object, either by holding steadily in the hand, or better 

 by a clamp attached, by which it can be screwed into a tree or 

 post, or fixed in the muzzle of a rifle. The instrument is turned 

 on the barrel of the glass till the sun is in the plane passing 

 through the two axes of the instrument, antl the mirror A is 

 turned till the bright image of the sun is seen on the screen //, 

 through a hole left for the purpose in the side of the tube. On 

 looking through the glass the sun's image is seen, and by then 

 slightly rotating the instrument or moving the mirror, is made to 

 cover the object. The mirror A is connected not directly to the 

 body of the instrument, but to a lever B, on which it works 

 stiflly, so as to retain any position in which it is placed. Lever 

 B works easily and has a limited range of motion, to one end of 

 which it is pressed by a spring ; slight pressure with the finger 

 moves it and its attached mirror, so as to throw tlie light on and 

 off the object in a succession of long and short flashes by which 

 letters and words may be indicated. Flashes may also be given 

 by moving the instrument if held in the hand. 



The above instrument answers well for all positions of the 

 sun except when very low behind the observer's back. For 

 this case another mirror is provided by which the light is 

 reflected on to the mirror A. 



The instrument, which is made by Cook and Sons, York, was 

 exhibited. It was favourably criticised by the president. Prof. 

 W. G. Adams, F. Galton, and others. 



SELENIUM AND THE PHOTOPHONE ' 



T N bringing before you some discoveries made by Mr. Sumner 

 Tainter and myself, which have resulted in the construction 

 of apparatus for the production and reproduction of sound by 

 means of light, it is necessary to explain the state of knowledge 

 which formed the starting poi]it of our experiments. I .shall 

 first describe the remarkable substance selenium, and the manipu- 

 lations devised by various experiments ; but the final result of our 

 researches has evidenced the class of substances sensitive to light 

 vibrations, until we can propound the fact of sensitiveness being 

 a general property of all matter. We have found this property 

 in gold, silver, platinum, iron, steel, brass, copper, zinc, lead, 

 antimony, German silver, Jenkin's metal, Babbitt's metal, ivory, 

 celluloid, gutta-percha, hard rubber, soft vulcanised rubber, 

 paper, parchment, wood, mica, and silvered glass ; and the only 

 substances from which we have not obtained results are carbon 

 and thin microscopic glass. We find that when a vibratory 

 beam of light falls upon these substances they emit sounds — the 

 pitch of which depends upon the frequency of the vibratory 

 change in the light. We find further that, when we control the 

 form or character of the light-vibration on selenium, and 

 probably on the other substances, we control the quality of the 

 sound and obtain all varieties of articulate speech. We can 

 thus, without a conducting wire as in electric telephony, speak 

 from station to station, wherever we can project a beam of light. 

 We have not had opportunity of testing the limit to which this 

 photophonic inflnence can be extended, but we have spoken to 

 and from points 213 metres apart ; and there seems no reason to 

 doubt that tlie results will be obtained at whatever distance a 

 beam of light can be flashed from one observatory to another. 

 The necessary privacy of our experiments hitherto has alone 

 prevented any attempts at determining the extreme distance at 

 which this new method of vocal communication will be available. 

 I shall now speak of selenium. 



In the year 1S17 Berzelius and Gottlieb Gahn made an 

 examination of the method of preparing sulphuric acid in 

 use at Gripsholm. During the course of this examination, 

 they observed in the acid a sediment of a partly reddish, 

 partly clear brown colour, which, under the action of the 

 blowpipe, gave out a peculiar odour, like that attributed 

 by Klaproth to tellurium. As tellurium was a substance of 

 extreme rarity, Berzelius attempted its production from this 

 deposit ; but he was unable, after many experiments, to obtain 

 further indications of its presence. lie found plentiful signs of 

 sulphur mixed with mercury, capper, zinc, iron, arsenic, and 

 lead, but no trace of tellurium. It was not in the nature of 

 Berzelias to Ije disheartened by this result. In science every 

 failure advances the boundary of knowledge as w-ell as every 

 success, and Berzelius felt that, if the characteristic odour that 

 had been observed did not proceed from tellurium, it might pos- 

 sibly indicate the presence of some substance then unknown to 

 the chemist. Urged on by this hope he returned with renewed 

 ardour to his work. He collected a great quantity of the mate- 

 rial, and submitted the w hole mass to various chemical processes. 

 He succeeded in separating successively the sulphur, the mer- 

 cury, the copper, the tin, and the other known substances whose 

 presence had been indicated by his tests; and, after all these 

 had been eliminated, there still remained a residue which proved 

 upon examination to be what he had been in search of — a new 

 elementary substance. The chemical properties of this new 

 element were found to reemble those of tellurium in so remark- 

 able a degree, that Berzelius gave to the substance the name of 

 "Selenium," from the Greek word se/eiie, the moon ("tellu- 

 rium," as is well known, being derived from tdhis, the earth). 



Although tellurium and selenium are alike in many respects, 

 they differ in their electrical properties, tellurium being a good 

 conductor of electricity, and selenium, as Berzelius showed, a 

 nonconductor. Knox discovered, in 1S37, that selenium became 

 a conductor when fused ; and Hittorff, in 1S52, showed that it 

 conducted, at ordinary temperatures, when in one of its allotropic 

 forms. When selenium is rapidly cooled from a fused condition 

 it is a non-conductor. In this its vitreous form it is of a dark 

 brown colour, almost black by reflected light, having an exceed- 

 ingly brilliant surface. In thin films it is transparent, and ap- 

 pears of a beautiful ruby red by transmitted light. When 

 selenium is cooled from a fused condition with extreme slowness 

 it presents an entirely different appearance, being of a dull lead 



' Lecture delivered at the Boston meeting o£ the American .Association by 

 Prof. A. Graham Bell. 



