58 



Captain W. de W. Ahney 



[March 17, 



only refer to Lamansky's tliermogram (Fig, 1) of the solar spectrum, 

 taken by a linear thermopile, in which, after taking extraordinary 

 precautions, he found the existence of three breaks. The thermo- 

 pile, with its galvanometer, takes no cognizance of time ; given a 

 constant and steady flux of radiation striking it during a certain 

 time, the electrical current generated, which is shown by the deflection 

 of the galvanometer needle, remains constant, and no increase of time 

 gives a greater deflection. If we could imagine a thermopile the 

 current generated by which, by the efiiux of time, would j^roportionally 

 increase the deflection of the needle, then the most limited radiation 



Fig. 1. 



striking the pile could be measured and calculated. At present 

 there seems to be no method capable of taking account of time for 

 this purpose except photography, and until recently it seemed chi- 

 merical to apply it. I should like to show you how photography 

 takes cognizance of time, and also why it seemed unsuitable for in- 

 vestigating the dark rays below the red end of the sj^ectrum. I propose 

 to photograph the spectrum on an ordinary photographic compound. 



A piece of paper has been coated with silver bromide, and this 

 compound, w^hen placed in the spectrum, is acted upon by the ultra 

 violet, the violet and the blue rays. If I cover up | of the slit of the 

 lantern and allow an exposure of the paj)er to the spectrum of two 

 seconds, then with the next ^ of the slit an exposure of ten seconds, 

 and with the remaining }^- give an exj)osure of thirty seconds, it will be 

 seen when I develop the image that the length of the spectrum varies, 

 and also the darkening of the different portions. Thus the longest 

 exposure wall show the greatest intensity of action and the greatest 

 length of spectrum. [Shown.] Tliis experiment serves a double 

 purpose, for whilst it makes it clear that photography takes cognizance 

 of time, yet it seemingly shows that it is unfitted for exploring the 

 infra-red region, since ordinary photographic compounds are unacted 

 upon by them. Could a compound be found which was sensitive to 

 the dark rays, it is manifest that the battle would be won, and that 

 investigations full of interest might be the outcome. 



