1910] The Telegraphy of Photographs, Wireless and hy Wire. 835 



WEEKLY EVENING MEETING, 



Friday, April 22, 1910. 



His Grace the Duke of Northumberland, K.G. P.O. D.O.L. 

 LL.D. F.R.S., President, in the Chair. 



T. Thorne Baker, Esq. F.C.S. A.I.E.E. 



The Telegraphy of Photographs, Wireless and hy Wire. 



It frequently happens that when two alternate processes are 

 available for certain work, and one of them is considerably less 

 practical than the other, the less practical one is possessed of much 

 higher scientific interest. This may certainly be said of the 

 telegraphy of pictures and photographs. The whole of the methods 

 of transmission can be classed as either purely mechanical, or de- 

 pendent on the physical properties of some substance which, like 

 selenium, is sensitive to light. 



The latter methods are of no little scientific interest, and, although 

 very delicate and for the moment obsolete, there is every likelihood 

 of their coming into more extended use later on. 



The telegraphy of pictures differs only from the transmission of 

 ordinary messages in that the telegraphed signals, recorded by a 

 marker on paper, must essentially occupy a fixed position. In the 

 case of an ordinary telegram it matters httle whether the received 

 message occupy two, three or more lines when written out on paper, 

 but when a picture is telegraphed every component part of it must 

 be recorded in a definite position on the paper. 



Suppose you greatly enlarge a portrait, and divide it up by ruled 

 lines into a thousand square parts. Suppose also that the photograph 

 is printed on celluloid, so that it is transparent. If, now, the portrait 

 be held in front of some even source of illumination, it will be seen 

 that each square — each thousandth part — is of different density. 

 The light parts of the photograph will consist of squares of little 

 density, the dark parts, of squares of greater density, and so on. 

 In this way the photograph is analysed into composite sections, each 

 section corresponding precisely to a letter in a message ; letters and 

 spaces recombined form words and messages ; squares of different 

 densities recombined, in correct position, form a photograph. 



I propose to deal with the more practical system first, which, as 

 already pointed out, is perhaps the less interesting from the theo- 

 retical point of view. The telectrograph system has been employed 

 by the ' Daily Mirror ' for the transmission of photographs since 

 July 1909, and has been worked very regularly between Paris and 

 London, and Manchester and London. 



