Feb. lo. 1881 



NA TURE 



345 



the line became at once diminished. Lastly, a screen of 

 black paper, having a large pin-hole in the middle, was 

 placed at a short distance before the selenium, and the 

 image of a gas- flame was focussed upon the outer surface 

 of the screen, a small portion of the hght passing through 

 the pin-hole and forming a luminous disk upon the 

 selenium. The galvanometer was again brought to zero, 

 and, as before, the platinum point made no mark. When 

 however the gas-flame was shaded a firm and steady line 

 could be drawn ; and when the light w^as interrupted by 

 moving the fingers before the pin-hole a broken line was 

 produced. For this last operation a very sensitive paper 

 was required, and it was found necessary to move the 

 platinum point slowly. 



In consequence of the very satisfactory results of these 

 prehminary experiments I made a pair of " tele-photo- 

 graphic " instruments, of which the receiver was slightly 

 modified from Bakewell's form. They are of rude con- 

 struction, and I shall say nothing more about them except 

 that on January 5 they produced a " tele-photograph" of 

 a gas-flame, which was good enough to induce me to 

 make the more perfect apparatus now to be described. 



The transmitting instrument consists of a cylmdrical 

 brass box four inches in diameter and two inches deep, 

 mounted axially upon a brass spindle seven inches long, 

 and insulated from it by boxwood rings. The spindle is 

 divided in the middle, its two halves being rigidly con- 

 nected together by an insulating joint of boxwood. One 

 of the projecting ends of the spindle has a screw cut upon 

 it of sixty-four threads to the inch ; the other end is left 

 plain. The spindle revolves, like that of a phonograph, 

 in two brass bearings, the distance between which is equal 

 to twice the length of the cylinder ; and one of the 

 bearings has an inside screw corresponding to that upon 

 the spindle. At a point midway between the two ends of 

 the cylinder a hole a quarter of an inch in diameter is 

 drilled, and behind this hole is fi.xed a selenium cell, the 

 two terminals of which are connected respectively with 

 the two halves of the spindle. The bearings in which the 

 spindle turns are joined by copper wires to two binding 

 screws on the stand of the instrument. The transmitter 

 thus described is represented in diagrammatic section at Y 

 (Fig. 2), where H is the hole in the cylinder and S the 

 selenium cell. 



The receiving instrument, shown at X (Fig. 2) contains 

 another cylinder similar to that of the transmitter, and 

 mounted upon a similar spindle, which however is not 

 divided, nor insulated from the cylinder. An upright 

 pillar D, fixed midway between the two bearings, and 

 slightly higher than the cylinder, carries an elastic brass 

 arm fitted with a platinum point P, which presses normally 

 upon the surface of the cylinder. To the brass arm a 

 binding screw is attached, and a second binding screw in 

 the stand is joined by a wire to one of the brass bearings. 

 To prepare the instruments for work they are joined up 

 as shown in Fig. 2, two batteries, a set of resistance coils, 

 and a galvanometer being used, in exactly the same 

 manner as in the preliminary experiments. The cylinder 

 of the transmitting instrument v is brought to its middle 

 position, and a picture not more than two inches square 

 is focussed upon its surface by the lens L. The pictures 

 upon which I have operated have been mostly simple 

 geometrical designs cut out of tinfoil and projected by a 

 magic lantern. It is convenient to cover a portion of 

 the cylinder with white paper to receive the image. The 

 comparatively large opening H is covered with a piece of 

 tin-foil, in which is pricked a hole which should be only 

 just large enough to allow the instrument to work. [I 

 have not been able to reduce it below one-twentieth of an 

 inch, but with a more sensitive selenium cell it might with 

 advantage be smaller.] The hole is then brought, by 

 turning round the cylinder, to the brightest point of the 

 picture, and a scrap of sensitised paper, in the same con- 

 dition as that to be used, being placed under the point P 



of the receiver, the resistance R is adjusted so as to bring 

 the galvanometer to zero. When this is accomplished 

 the two cylinders are screwed back as far as they will go, 

 the cylinder of the receiver is covered with sensitised 

 paper, and all is ready to commence operations. 



The two cylinders are caused to rotate slowly and 

 synchronously. The pin-hole at H in the course of its 

 spiral path will cover successively every point of the 

 picture focussed upon the cylinder, and the amount of 

 light falling at any moment upon the selenium cell will 

 be proportional to the illumination of that particular spot 

 of the projected picture which for the time being is occu- 



FiG. 3- — Im.ige focussed upon Tmnsmitter. 



pied by the pin-hole. During the greater part of each 

 revolution the point P will trace a uniform brown line ; 

 but when H happens to be passing over a bright part of 

 the picture this Ime is enfeebled or broken. The spiral 

 traced by the point is so close as to produce at a little 

 distance the appearance of a uniformly-coloured surface, 

 and the breaks in the continuity of the line constitute a 

 picture which, if the instrument were perfect, would be a 

 monochromatic counterpart of that projected upon the 

 transmitter. 



An example of the performance of my instrument is 

 shown in Fig. 4, which is a very accurate representation 

 of the manner in which a stencil of the form of Fig. 3 is 

 reproduced when projected by a lantern upon the trans- 

 mitter. I have not been able to send one of its actual 

 productions to the engraver, for the reason that they are 

 exceedingly evanescent. In order to render the paper 

 sufficiently sensitive, it must be prepared with a very 

 strong solution (equal parts of iodide and water), and 

 when this is used the brown marks disappear completely 



Fig. 4. — Image as reproduced by Receiver. 



in less than two hours after their formation. There is 

 little doubt th.at a solution might be discovered which 

 would give permanent results with equal or even greater 

 sensitiveness, and it seems reasonable to suppose that 

 some of the unstable compounds used in photography 

 might be found suitable; but my efforts in this direction 

 have not yet been successful. 



In case any one should wish to repeat the experiments 

 here described a few practical hints may be useful. In 

 order that as large a portion as possible of the current 

 from the battery u' (which is varied by the selenium cell) 



