THE BEGINNINGS OF PHOTOGRAPHY. 3-19 



He also used colored ribbons or papers in ditferent thicknesses to grad- 

 uate the amount and color of the light falling on the woods, and thus 

 appears to have been the first to use a ])hotometer. He foiuid that 

 the change of color was due to the resinous constituents of the wood 

 and the liability to change depended on the amount of resin present. 

 He does not seem to have made any o])servations on changes in the 

 solu))ility of resins after exposure to light, but apparenth' recognized 

 that the light brought about an oxidation, and that some resins are 

 bleached while others are darkened. 



In the fourteenth memoir (Vol. HI, p. 184) lie discusses the action 

 of light on mineral substances and, after Ijriefl}- noting its action on 

 several metallic compounds, he deals, in section 3, page 192, with 

 the compounds of silver, and especially the chloride (luna cornea). 

 He refers to the previous work of Beccari, Meyer, Schulze, and 

 Scheele, and proposes to extend it. Not content with the simple 

 experiment with a cut-out stencil of sheet brass, he instituted a series 

 of pliotometric observations, b}^ exposing the chloride under a varying 

 number of thicknesses of paper or slips of difierent woods or of glass. 

 He confirmed Scheele's experiment with the spectrum and extended 

 it by measuring the length of time it took each ray to darken the 

 chloride, and found that while the more refrangible rays at the blue 

 end only took from fifteen to thirty-five seconds the less refrangible 

 from the yellow to the red required from five and one-half to twenty 

 minutes. 



Valuable and interesting as Senebier's observations are as a contri- 

 bution to the science of vegetable physiology, they did not advance 

 photography very much, except in so far as they marked the intro- 

 duction of a system of photometric measurement, which in recent 

 years has been recognized as the only reliable basis of scientific 

 photographic investigation, and perhaps no one has contributed more 

 to this than our esteemed president, Sir William Abney. 



BERTHOLLET, 



In the Memoirs of the French Academy for 1785 Berthollet pulj- 

 lished some researches on the action of light ujion plants, etc., as well 

 as upon silver chloride under water, and attributed the darkening of 

 the silver salt to a partial reduction of the metal caused by the disen- 

 gagement of oxj'gen loosely combined with it. He explains this as 

 being conformable to a law of afiinit}' under which the adherence of 

 any element increases in proportion as its quantity grows less, and 

 remarks that gold, silver, and mercury are precipitated on animal 

 substances in this medium state between the oxide and the metal. 

 Doctor Eder notes that Berthollet was thus the first to suggest the 

 formation of a subchloride or oxysubchloride by the action of light 

 on silver chloride. Berthollet's views were afterw^ards chaue-ed more 



