A Manual Of Photography

150 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. that the whole plate had been previously by accident slightly affected by light, and that the exposure through the holes to another sort of light had destroyed the former effect I was naturally led to that explanation, having before observed that one kind of light destroys the effect of another; as, for examp e, that the effect of the light from the north is destroyed by the light from the south, when certain vapours existing in the lattei portion of the atmosphere impart a yellow tint to the light of the sun. But after repeated experiments, taking great care to protect the plate from the least exposure to light, and recol- lecting some experiments of M. Moser, (see Chapter on Ther- mography,) I found that the affinity for mercury had been im- parted to the surface of the daguerreotype plate by the contact of the metallic plate having the round holes, while the space under the hole had received no similar action. But it must he observed that this phenomenon does not take place every time some days it is frequent, and in some others it manifest itself at all. Considering that the plate furnished with round holes is of copper, and that the daguerreotype plate is ot silver plated on copper, it is probable that the deposit ot mercury is due to an electric or galvanic action determined ^îhe contact of the two metals ; and perhaps the circumstance that the action does not take place every time, will lead to the supposition t ia it is developed by some peculiar electric state of the ambient atmosphere ; and by a degree of dampness in the air which would increase the electric current. May we not hope that the conditions being known in which the action is produced, and by availing ourselves of that property, it will be possible to increase on the daguerreotype plate the action of light ? for it is not improbable that the affinity for mercury imparted to the plate is also due to some electrical influence of light. How could we otherwise explain that affinity for mercury given by some rays and withdrawn by some others, long before light has acted as a chemical agent ? , . . -. . tin a “ The question of the actinic focus is involved in another kind of mystery, which requires some attention I KE found that with the same lenses there exists a constant variation in the distance between the two foci. They are never in the same relation to each other : they are sometimes more or less sepa- rate ; in some lights they are very distant, and in some oth they are very near, and even coincide. For this reason I con- stantly try their position before I operate. 1 have not beeil able to discover the cause of that singular phenomenon, but I can state positively that it exists. At first, 1 thought that some variations in the density or dispersive power of the atmosphere