THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 

 NOVEMBER 1848. 



XLIX. Description of the Photographometer, an instrument for 

 measuring the Intensity of the Chemical Action of the Rays of 

 Eight on all the Photographic preparations, and for com- 

 paring with each other the sensitiveness of these different pre- 

 parations. Invented by A. Claudet, London, September 

 1 848 *. 



[With a Plate.] 



WE are not yet in possession of an instrument capable of 

 measuring the variations in intensity of the sun's light 

 with the same precision that we appreciate those of its heat. 

 In photographic operations the want of a photometer has long 

 been felt ; and it was soon perceived that it was not sufficient 

 to measure the light, properly so called, but certain accom- 

 panying rays, which exert on different bodies a chemical action 

 without producing on the retina the sensation by means of 

 which objects appear to us visible. All photographers know 

 by expei'ience that the chemical action is not exactly in pro- 

 portion to the intensity of the visible light. 



Many ingenious contrivances have been resorted to for in- 

 dicating the intensity of the chemical or actinic rays existing 

 at a given moment ; but as the object was to study more par- 

 ticularly the variations of the direct solar rays, it was found 

 necessary to have recourse to complicated instruments fur- 

 nished with heliostats and clock-work movements. 



In 1839 Mr. T. B. Jordan of Falmouth invented an instru- 

 ment which he called a heliograph. This instrument was 

 imperfect. It was modified and improved by Mr. R. Hunt, 

 who has devoted much time to photographic processes and 

 researches on the properties of the different rays of light. 



The apparatus of Mr. Hunt, which he called an actino- 

 graph, consists of a cylinder placed parallel to the axis of the 



* This paper has been communicated to the Academie des Sciences, 

 October 9. 



Phil. Mag. S. 3. Vol. 33. No. 223. Nov. 1848. Z 



