TRANSACTIONS OF THE SECTIONS. 35 
solution of gallic acid, containing 20 grs. in the fluid ounce, to which a little strong 
acetic acid is added, and is then fixed with a solution of hyposulphite of soda in 
the usual manner. The present improvement consists in rinsing the paper in water 
after the application of the solution of nitrate of silver, pressing out the superfluous 
moisture in folds of blotting-paper, and then adding a little more of the solution of 
nitrate of silver to the surface of the paper; this is most conveniently effected by pour- 
ing a small quantity on the paper, and then passing a glass rod or tube lightly over 
the paper, by which the solution is evenly distributed over the surface, and the con- 
tact of organic matter avoided. The increased sensibility and improved cleanliness 
of the paper consequent on this addition to the process, are presumed to depend on 
the removal by washing of the nitrate of potash formed by the mutual decomposition 
of the salts on the surface of the paper. 


Researches on the Theory of the principal Phenomena of Photography in the 
Daguerreotype Process. By A. CuauveErt. 
% The various questions treated by M. Claudet were the following :— 
. 1. What is the action of light on the sensitive coating ? 
3 2. How does the mercurial vapour produce the Daguerreotype image ? 
bo 3. Which are the particular rays of light that impart to the chemical surface the 
Ee affinity for mercury? 
a 4, What is the cause of the difference in achromatic lenses between the visual and 
photogenic foci? Why do they constantly vary ? 
5. What are the means of measuring the photogenic rays, and of finding the true 
focus at which they produce the image? 
Light produces two different effects on the Daguerreotype plate capable of giving 
animage. By one the surface is decomposed, and the silver is precipitated as a white 
powder; this action is very slow. By the other, the parts affected by light receive 
an affinity for the mercurial vapour, and this metal is deposited in white crystals. 
ee This action, which is the cause of the Daguerreotype image, is 3000 times more rapid 
_ than that producing the decomposition of the surface. After having examined the 
__ phznomena of these two actions, M. Claudet considers that it is impossible to refer 
_ them to the same cause. The first is a chemical decomposition of the surface, and 
4 the second is a mere new property imparted to the surface to attract the vapour of 
_ mercury, which is given by some particular rays and withdrawn by some other rays. 
_ The most refrangible rays produce the affinity for mercury, and the least refrangible 
_ withdraw it. 
M. Claudet afterwards explained the principle of his photographometer, and 
_ several improvements he has lately made in that instrument, by which he can com- 
re upon the same plate a series of intensities in a geometrical progression, varying 
m 1 to 512, and when employing two plates at the same moment, from 1 to 8192; 
and by another modification of the instrument—by shutting one-half of every hole 
_ through which the light has affected the plate, and submitting this half to radiation 
through red, orange or yellow glasses—he can study the modifications produced on 
these various intensities of effect, by these coloured or insulated radiations, The ex- 
periments to which M. Claudet refers would be too long to enumerate here, and 
_ we shall conclude by alluding to the most important point of this paper, which is the 
question of the difference between the visual and photogenic foci in achromatic 
__ lenses, and the constant variations they undergo by the influence of unknown causes, 
at all events, which he has not been able to ascertain. It is known that several years 
ago M. Claudet was the first to point out the difference between the two foci, and 
the necessity for the operator to place exactly the plate at the point where the pho- 
_ togenic focus is produced, in order to have a correct Daguerreotype image. But 
__ the new important fact lately observed by M. Ciaudet refers to the constant variation 
_ between the proportionate distance of these two foci. It appears that, according to 
Some causes which M. Claudet has not been yet able to discover, the two foci for 
_ the same distance of an object are sometimes coinciding, and sometimes very far, 
_ one from the other; and what is most remarkable is, that the difference varies ac- 
_ eording to some properties of the lenses, in such a manner that when the two foci 
_ coincide in one case they may be very much separated in the other. M 
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