412 THE SCIENTIFIC PAPERS OF 



indications were visible to a number of persons at once. [Dr. 

 Siemens then experimented with variously- coloured sheets of card- 

 board prepared for the purpose, and the reflected light was found 

 to cause a deflection of the galvanometer in each case, the slightest 

 effect being produced with light reflected from a black piece of 

 paper, and successively increasing with green, red, and white, the 

 greatest of all being produced by exposing it to the direct light of 

 an argand burner.] These experiments showed the great sensi- 

 tiveness of selenium ; but Professor Bell had gone much further, 

 and had prepared an instrument with concentric plates of selenium 

 and intervening plates of mica, and operating upon a much larger 

 surface. He had gone much further than had been done previously. 

 Then came the further step which he had so boldly taken, of 

 making light become the carrier of speech. As he had justly 

 said, this seemed marvellous at first, but when you knew how to 

 do it, it became simple, like everything else, and he (Dr. Siemens) 

 must congratulate the Society on having had the method of doing 

 it so clearly explained. 



In the discussion of the Paper 



" ON THE WEIGHT AND LIMITING DIMENSIONS OF 

 GIRDER BRIDGES," by MAX AM ENDE, Assoc. M. Inst. C.E., 



DR. SIEMENS * said, though the subject was one rather of con- 

 struction of bridges, than of the durability or mode of treatment 

 of materials, he thought that a few observations might not be 

 inappropriate with reference to the remark of Mr. Bender, to the 

 effect that steel gave way with a strain of 60 per cent, of the cal- 

 culated strain. Steel was essentially a different material from 

 iron. The difference was like that between parchment and woven 

 fabric. In iron there were fibres which acted separately, so to- 



* Excerpt Minutes of Proceedings of the Institution of Civil Engineers, Vol. 

 LXIV. Session 1880-81, pp. 289-290. 



