378 Mr. A. Claudet on the pHficipal Phenomena of 



atmosphere some vapours which absorb the most refrangible 

 rays. In these circumstances the light appears rather yellow ; 

 but it is very difficult to judge by the eye of the exact colour 

 of the light, and of the proportion of photogenic rays existing 

 in the atmosphere at any given moment. 



The vapours of the atmosphere which render the light yel- 

 low, act as an}' other medium intercepting the blue rays, and 

 those which have the same degree of refrangibility. I prove, 

 by a very simple experiment, the comparative photogenic 

 action of rays which have passed through such media, and of 

 those which have met with no similar obstacle; also that media 

 which intercept the photogenic rays can let pass freely the 

 illuminating rays. 



If I cover an engraving one-half with light yellow glass, and 

 place it before my camera obscura in order to represent the 

 whole on a Daguerreotype plate, I find that during the time 

 which has been necessary to obtain the image of the half not 

 covered, not the slightest effect has been produced on the half 

 covered with the yellow glass. 



Now if I cover one half with deep blue glass and the other 

 with the same light yellow glass, the engraving will be seen 

 very distinctly through the yellow glass, and not at all through 

 the blue. In representing the whole, as before, on the Da- 

 guerreotype plate, the half which was clearly seen by the eye 

 has produced no effect; and the other, which could not be 

 seen, is as fully represented, and in nearly as short a time, as 

 when no blue glass had been interposed. 



Thus we might construct a room lighted only through an 

 inclosure of light yellow glass, in which light would be very 

 dazzling to the eye, and in this room no photographic opera- 

 tion could be performed ; or a room inclosed by deep blue 

 glass, which would appear very dark, and in which the pho- 

 tographic operation would be nearly as rapid as it would be 

 in open air. 



Thus we ma}' conceive certain states of the atmosphere 

 under which there will be an abundance of illuminating rays, 

 and very few photogenic rays ; and some others, under which 

 the reverse will take place. 



Considering how difficult it is to judge by the eye alone 

 of the photogenic state of light, we can understand why the 

 photographer is constantly deceived in the effect he tries to 

 produce, having no means to ascertain beforehand, with any 

 degree of certainty, the intensity of light. For these reasons 

 I turned my attention to contrive an apparatus by which 

 I could test at the same time the sensitiveness of the Daguer- 

 reotype plate and the intensity of light. 



