A Manual Of Photography

ON THE THEOBY OF THE BAGUEBBEOTYPE. 147 sity 3000 times greater ; it is the kind of action produced by an intensity 3000 times less, giving the affinity for mercury, which is completely destroyed by the red, orange, or yellow rays. White light, or the chemical rays which accompany it, communi- cate to the surface the affinity for mercury ; and the red, orange, or yellow rays withdraw it. This is in effect the same pheno- menon as Dr. Wollaston observed with the tincture of gum guaiacum ; one set of rays restoring the colour which another set had removed. A singular anomaly requires notice : viz. that when the sensitive surface is prepared only with iodine without bromine, the red, orange, or yellow rays, instead of destroying the action of white light, continue the effect of decomposition as well as that of affinity for mercury. Still there is a double compound of iodine which is far more sensitive than the simple compound, and on whicli the red, orange, or yellow rays exercise their destructive action as in the case of the bromo-iodide. The phenomenon of the continuing action of the red, orange, or yellow rays, on the simple compound of iodide of silver, was discovered by M. Ed. Becquerel ; and soon after M. Gaudin found, that not only those rays continue the action by which mercury is deposited, but that they develope without mercury an image having the same appearance as that produced by mer- curial vapour. M. Gaudin, not having observed the fact of the white precipi- tate, which is the result of the decomposition by the action of light, could not explain the cause of the image brought out under the influence of the yellow ray. M. Claudet states that the iodide of silver without bromine is about 100 times more sensitive than the bromo-iodide to the action of the rays whicli produce the decomposition of the com- pound forming the white precipitate of silver, while it is 100 times less sensitive for the effect whicli gives the affinity for mercury. It may be that, in the case of the iodide of silver alone, the decomposition being more rapid, and the affinity for mercury slower than when bromine is added to the compound, the red, orange, or yellow rays having to act upon an incipient decomposition, have the power, by their own photogenic influence, of continuing the decomposition when it has begun. This may explain the development of the image under red, orange, or yellow glasses, according to M. Gaudin's discovery. But in'the case of the bromo-iodide of silver, the red, orange, or yellow rays have to exert their action on the affinity for mercury, begun a long time before the decomposition of the compound; and they have the property of destroying that affinity. So that it would appear that all the rays of light have the pro-