August 29, 1907J 



NA TURE 



445 



its spiral path the pinhole H covers successively every 

 point of the focussed image, the illumination of the 

 Se cell being proportional at any moment to the 

 brightness of the spot occupied by the pinhole ; the 

 consequent variation in the resistance of the cell 

 causes the stylus F to trace upon the paper a brown 

 line which is lighter or darker in correspondence with 

 the illumination of the Se. The close spiral line with 

 breaks in its uniformilv constitutes a picture, which 

 should be a counterpart of that projected upon T. 

 The earliest achievement of the apparatus consisted 

 in the reproduction of the image of a hole cut in a 

 piece of black paper ; after some improvements simple 

 black and white pictures painted upon glass were very 

 perfectly transmitted, as was demonstrated upon 

 several occasions when the apparatus was exhibited 

 in operation.' It was, however, unable to cope with 

 half-tones, and owing to pressure of vi'ork the experi- 

 ments were shortly afterwards discontinued. 



The problem of telegraphic photography has re- 

 cently been attacked with conspicuous success by 

 Prof. A. Korn, of Munich, whose work is described 

 in a little book entitled " Elektrische Fernphoto- 

 graphie und Ahnliches " (Leipzig, 1907). His latest 



Fig. I. 



method is indicated in Fig. 2. The transmitting and 

 receiving cylinders T, R turn synchronously on 

 screvi'ed axes, the regulating mechanism of the re- 

 ceiver is situated in the bridge C D, and a suitable 

 resistance is placed at S,. .\ celluloid film negative 

 of the picture to be transmitted is wrapped round 

 the cylinder T, which is made of glass. The light 

 of a Nernst lamp Nj is concentrated by a lens 

 upon an element of the film, through which it 

 passes more or less freely according to the trans- 

 lucency of the film at the spot, to the Se cell S,, which 

 is fixed in position, and does not, like mine, move 

 with the cylinder; thus the resistance of the Se is 

 varied in correspondence with the lights and shades of 

 the picture. The receiving cylinder R is covered with 

 a sensitised photographic film or paper, upon a point 

 of which light from a lamp N, is concentrated. Be- 

 fore reaching the paper the light passes through 

 perforations in two iron plates at F, which are, in 

 fact, the pole-pieces of a strong electromagnet ; between 

 these is a shutter of aluminium leaf, which is at- 

 tached to two parallel wires or thin strips forming the 



1 Among others, al the Telegraph Engineers' soirie in iS8i (see Nature, 

 vol. xxiii., p. 563). 



bridge C D. When there is no current through 

 C D, the opening is covered by the shutter; when a 

 current traverses the wires, they are depressed by 

 electromagnetic action, carrying the shutter with 

 them, and a quantity of light proportional to the 

 strength of the current is admitted through the per- 

 forations. By means of this " light-relay," as it is 

 termed, the intensity of the light acting at any 

 moment upon the sensitised paper is made propor- 

 tional to the illumination of the selenium in the trans- 

 mitter. 



It remains to mention a device of admirable in- 

 genuity which has rendered it possible to transmit 

 half-tones with fidelity. In its response to changes 

 of illumination selenium exhibits a peculiar kind of 

 sluggishness, to which reference was made in my old 

 article : " Some alteration takes place almost in- 

 stantaneously with a variation of the light, but for 

 the greater part of the change an appreciable period 

 of time is required." Prof. Korn has succeeded in 

 eliminating the effects of the sluggish component by 

 substituting for mv box of resistance coils R a second 



Se cell S„, which is as nearly as possible similar to 

 S,, and which, by means of a second light-relay H, 

 placed in series with the first, is subjected to similar 

 changes of illumination. Thus any subpermanent 

 fall in the resistance of S, due to the action of light 

 is compensated by an equal fall in that of S,, and 

 only such changes as respond immediately to the 

 varying illumination of S, are utilised for regulating 

 the transmission current. 



Such is in brief outline the nature of the new pro- 

 cess. As regards the many carefully considered 

 details which have made it a practical success, those 

 interested will find ample information in the pamphlet 

 mentioned above. The apparatus has been worked 

 with excellent effect over long distances; a specimen 

 of its performance, for which 1 am indebted to the 

 kindness of Prof. Korn, is given in Fig. 3. The 

 parallel lines traced by the point of concentrated light 

 — in this case about 50 to the inch — are easily recog-- 

 nisable. 



Shelford Bidwell. 



NO. 1974, VOL. 76] 



