THE TELEGRAPHY OF PHOTOGRAPHS, WIRELESS AND 



BY WIRE. 1 



[With 2 plates.] 



By T. Thorne Bakeb, Esq., F. C. S., A. I. E. E. 



It frequently happens that when two alternate processes are avail- 

 able 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 transmis- 

 sion can be classed as either purely mechanical, or dependent on the 

 physical properties of some substance which, like selenium, is sensi- 

 tive 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 little 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 photo- 

 graph 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 analyzed into composite sections, each 

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



1 Lecture before the Royal Institution of Great Britain, at the weekly evening meeting, 

 Friday, Apr. 22, 1910. His Grace the Duke of Northumberland, K.G.. P.C., D.C.L., LL.D., 

 F.R.S., president, in the chair. Reprinted by permission from author's copy published by 

 the Royal Institution. Printed also in Nature, No. 2129, Aug. 18, 1010. 



97578°— sm 1910 17 257 



