334 Mr. A. Claudet's Description of the Photographometer. 



second yellow coating. The number of white circular spots 

 on each vertical zone indicates the degree of sensitiveness of 

 these various coatings ; the less sensitive being the first coat- 

 ing of yellow, and the most sensitive the second coating of the 

 same colour, which I call the double coating of iodine. 



Therefore, we have an instrument by which we can de- 

 cide, in an incontestable manner, upon the merits of different 

 accelerating liquids used in the Daguerreotype, resolve a great 

 number of problems, and make experiments, from which will 

 infallibly result the simplification and improvement of the art 

 of photography, which up to the present time has suffered too 

 much from empiricism and charlatanism. 



Appended to this description are a drawing of the apparatus, 

 and the plans necessary to explain its action and construction. 

 (See Plate II.) 



I am still engaged in making experiments with this appa- 

 ratus ; as soon as they are completed I shall hasten to com- 

 municate the result of my labours. I content myself for the 

 present with announcing a very extraordinary fact which my 

 apparatus has furnished me with. 



I do not give it as the result of a calculation mathematically 

 correct ; but I cannot be far from the truth in stating, that 

 the pure light of the sun modifies the bromo-iodized silver 

 plate, communicating to it an affinity for mercurial vapour 

 which produces the white image in the Daguerreotype, in a 

 space of time which cannot be much more than the thousandth 

 part of a second. I made the experiment in the following 

 manner : — I let the light of the sun fall upon the plate through 

 an opening of a millimetre, whilst this opening passed over a 

 space of 250 millimetres in one quarter of a second, as near as 

 I could judge ; this light could not therefore have acted on the 

 plate during much more than the y^^dth part of a second, 

 and nevertheless this inconceivably short space of time suf- 

 ficed to produce a decided effect. 



It is not necessary that I should indicate all the applica- 

 tions of this photographometer, and the experiments which 

 may be made with it. Without doubt many will be discovered 

 which have not occurred to me; but I will enumerate a few 

 which appear to me important enough to merit the attention 

 of philosophers : — What is the effect of the compound light, 

 and that of the different separated rays of the solar spectrum? 

 How much photogenic light is lost by reflexion from parallel 

 mirrors, prisms and other substances, and by refraction 

 through lenses? The proportion of photogenic rays in the 

 lights obtained from various sources, including that produced 

 by electricity ? If the photogenic light varies with the height 

 of the atmosphere and with the changes of temperature? If 



