June 4, 1908J 



NA TURE 



105 



TELEGRAPHIC PHOTOGRAPHY .LVD 

 ELECTRIC VISION. 



THE success achieved by Dr. A. Korn in the tele- 

 tjraphic transmission of photof;raphs (Nature, 

 vol. Ix.wi., p. 444) has been followed by a remarkable 

 development of inventive activity in the same line. 

 .Among several new processes which have recently 

 claimed public attention three of the most promising 

 were described in detail at the April meeting of the 

 Societe Frani,-aise de Physique. .As in Dr. Korn's 

 method, the reproduced picture is in all cases con- 

 stituted by a close spiral line of varying intensity 

 traced upon a photographic film, or other material, 

 covering a cylinder which rotates synchronously with 

 another cvlinder in the transmitting instrument. The 

 use of selenium for controlling the resistance of the 

 circuit is, however, generally discarded, the requisite 

 variations of current being effected by purely 

 mechanical means ; ordinary film negatives, therefore, 

 cannot be used. 



In M. E. Belin's process, which he calls " T^lestereo- 

 graphie," the original picture is a thickly coated 

 gelatin-bichromate print, which has the form of a 

 relief, lights being represented by elevations of the 

 surface, and shades by depressions. The print is 

 wrapped around the transmitting cylinder, which 

 rotates uniformly, and at the same time moves slowly 

 in a direction parallel to its axis. A sapphire point 

 attached to the short arm of a lever presses lightly 

 upon the picture, and is caused to move in corre- 

 spondence with the contour of the surface. At the end 

 of the long arm of the lever is a contact piece which 

 slides over the edges of a series of thin copper plates, 

 separated by sheets of mica, and connected with 

 resistance coils, the whole constituting a rheostat cap- 

 able of interposing in the circuit a resistance ranging 

 from o to 4000 ohms in 20 steps. The current, 

 regulated in accordance with the undulations tra- 

 versed by the sapphire point, passes at the receiving 

 station through an aperiodic reflecting galvanometer, 

 such as Blondel's oscillograph. A beam of light con- 

 centrated upon the galv'anometer mirror is reflected to 

 a convex lens so placed as to project an image of 

 the mirror over a small hole in the side of a light- 

 tight box, inside which rotates the receiving cylinder 

 covered with a photographic film. Between the hole 

 and the lens, and close to the latter, is inserted an 

 " optical wedge," consisting of a sheet of glass tinted 

 by gradations from perfect transparency at one end 

 to opacity at the other. A slight deflection of the 

 mirror displaces the reflected rays from the centre 

 of the lens towards the edge, and causes them to 

 pass through a different part of the optical wedge ; 

 thus the intensity of the projected image of the mirror, 

 and therefore of the photographic action upon the film, 

 is varied in correspondence with the strength of the 

 current. Photographs measuring 13 cm. by 18 cm. 

 were transmitted by this apparatus over a double-wire 

 telephone line in 22 minutes. 



For use in Carbonnelle's instrument, the " Tel^- 

 autograveur," the photograph is submitted to a treat- 

 ment, details of which are not published, whereby 

 the electrical conductivity is varied locally, being 

 greater in the shades than in the lights. The picture 

 is mounted upon the transmitting cylinder, and a 

 stylus bearing upon the surface is joined to one 

 of the line-wires. The receiving cylinder is covered 

 with a sheet of soft metal, gelatin or celluloid, or 

 simply by several sheets of white paper separated by 

 carbonic paper. The sheet is acted upon by a graving 

 point attached to the diaphragm of a telephone, which 

 is in circuit with the transmitting apparatus. This 

 process is said to give very satisfactory results, and 



NO. 2014, VOL. 78] 



to be remarkably rapid, a portrait 9 cm. by 7 cm. having 

 been reproduced over a distance of 90 kilometres in 88 

 seconds. 



.\ picture adapted for transmission by M. Pascal 

 Berjonneau's " Telephotographe " is prepared as a 

 cylindrical half-tone block, the surface consisting of 

 a multitude of metallic points, the density of which 

 varies with the lights and shades. The block rotates 

 under a stylus in the usual way, regulating the cur- 

 rent which passes over the line. At the receiving 

 station a galvanometer actuates a shutter which 

 allows more or less light to pass through a small 

 aperture to the photographic film. It is claimed for 

 this apparatus that it is the only one which can be 

 operated satisfactorily on an ordinary telegraph line, 

 all others requiring a telephone line with metallic 

 return. The time occupied in sending a portrait from 

 Paris to Enghien by a single wire is said to have 

 been 247 seconds. 



The problem of telegraphic photography is often 

 associated in the popular mind with that of distant 

 electric vision, or "seeing by electricity," as it has 

 been called. .According to a telegram from the Paris 

 correspondent of the Times, dated .April 28, the latter 

 pioblem is now engaging the attention of M. 

 .Armengaud, president of the French Society of Aerial 

 Navigation, who " firmly believes that within a year, 

 as a consequence of the advance already made by his 

 apparatus, we shall be watching one another across 

 distances hundreds of miles apart." It may be 

 doubted whether those who are bold enough to at- 

 tempt any such feat adequately realise the difficulties 

 which confront them. The telegram referred to seems 

 to contemplate the transmission of optical images over 

 an ordinary telegraph or telephone line by a method 

 in which advantage is taken of visual persistence. 

 A necessary condition would be that the sensitive sub- 

 stance — selenium or other photoelectric body — should 

 pass at least ten times per second over every unit 

 of the surface upon which the image to be transmitted 

 is projected, while at the distant station electrical con- 

 nection is established, also ten times per second, be- 

 tween the line wire and every individual element in 

 succession of the apparatus illuminating the receiving 

 screen; and the synchronism of the arrangement must 

 be so perfect that at the moment when the sensitive 

 substance occupies any given unit area on the surface 

 of the transmitter, connection must be made with the 

 corresponding unit in the receiver. The difficulty im- 

 posed by this condition depends chiefly upon the 

 necessarily large number of the units of area to be 

 dealt with. Suppose that the image is received upon 

 a screen no greater than 2 inches square ; if its 

 definition is to be as perfect as that presented by the 

 eye or by a good photograph, the number of elements 

 required would amount to about 150,000, and the 

 synchronised operations to a million and a half in 

 every second. If we are satisfied with a definition 

 equal to that of the coarse half-tone pictures to be 

 found in some of the daily newspapers, the necessary 

 number of elements might be reduced to 16,000, and 

 that of the synchronised operations to 160,000 per 

 second. Even this would be wildly impracticable, 

 apart from other hardly less serious obstacles which 

 would be encountered. The number of operations 

 might, of course, be greatly diminished by employ- 

 ing an oscillating or rotating arm, carrying a row 

 of' sensitive selenium cells, as was proposed by Profs. 

 Ayrton and Perry nearly thirty years ago. For a 

 coarse-grained picture 2 inches square 120 of these 

 might suffice ; but such an arrangement would require 

 120 line wires, and would also introduce a new series 

 of troubles. 



But although the problem is apparently incapable of 



