A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 149 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 ope- ration could be performed ;1 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 may conceive certain states of the atmosphere under which there will be an abundance of illuminating rays, and very few actinic rays ; and some others, under which the reverse will take place. Considering how difficult it is to judge by the eye alone of the chemical 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 ot light. 1or these rea- sons M. Claudet turned his attention to contrive an apparatus by which he could test at the same time the sensitiveness of the daguerreotype plate and the intensity of light. This instrument he called a Photographometer. “By this instrument," says the inventor, “ I have been able to discover at what degree of intensity of light the effect called solarization is produced: on well-prepared plates of bromo-iodide it does not begin under an intensity 512 times greater than that which determines the first effect of mercury ; and also at what degree the decomposition producing the white precipitate without mercury manifests itself, both on iodide and on bromo-iodide of silver. * On the first, it is 100 times quicker than on the bromo- iodide ; and on the last it is produced by an intensity 3000 times greater than that which developes the first affinity for mercury. ‘ 6 In the course of my experiments I noticed a curious fact, which proved very puzzling to me, until I succeeded in assigning a cause to it. I shall mention it here, because it may lead to some further discoveries. I observed that sometimes the spaces under the round holes, which had not been affected by light during the operation of the photographometer in a sufficient degree to determine the deposit of mercury, were, as was to be expected quite black ; while the spaces surrounding them were in an unaccountable manner slightly affected by mercury. At first I could not explain the phenomenon, except by supposing I j}iave recently proved that this statement requires some modification; the rays permeating yellow glasses act powerfully on the sensitive surfaces of collodion and iodine.