" Perhaps the most valuable practical application of these facts is in the use of the same plate for receiving several impressions. When, on taking the portrait or picture of any object liable to move, there is reason to suppose that the motion of the person or object has rendered the operation useless, it is not necessary to throw aside the plate on which the imperfect impression has been taken, and resort to the tedious process of cleaning and preparing another ; it is only necessary to treat the plate in the manner already pointed out, and it is again equal in every respect to a newly-prepared plate; and this treatment may be repeated, until, by the slow accumulation of too thick a film of iodide of silver, the plate no longer possesses the same degree of sensitiveness to light".

The researches of M. Claudet are of considerable importance, particularly as being those of a thoroughly practical photographic artist.

The phenomena which M. Claudet considers have not yet been satisfactorily explained, and of which he treats, are those referring to the following points :

1. Which is the action of light on the sensitive coating ?

2. How does the mercurial vapour produce the daguerreotype image ?

3. What are the particular rays of light that impart to the chemical surface the affinity for mercury ?

4. Which is the cause of the difference in achromatic lenses between the visual and photogenic lenses ? 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 1.

At the meeting of the British Association at Swansea, M. Claudet expressed his opinion that the decomposition of the chemical surface of the daguerreotype plate, by the action of certain rays of light, produced on that surface a white precipitate, insoluble in the hyposulphite of soda, which, when examined by the microscope, had the appearance of crystals reflecting light, and which, when seen by the naked eye, were the cause of a positive daguerreotype image. These were probably particles of pure white silver.

The opinion of Daguerre himself, and other writers, was, that the action of light on the iodide of silver had only the effect of darkening the surface, and consequently of producing a negative image. But it escaped them, that, under the darkened iodide of silver, another action could take place after a continued exposure to light, and that the hyposulphite of soda washing could disclose a positive image. M. Claudet proved this fact in obtaining, by the action of light only, and without mercury, images having the same appearance as those developed under the action of mercurial vapour. This direct and immediate effect of light is certainly remarkable ; but the daguerreotype process is not founded on that principle, on account of the slowness of its action; and it is fortunate that, long before light can produce the white precipitate alluded to, it operates another effect, which is the wonderful property of attracting the vapour of mercury. This vapour is condensed in the form of a white powder, having also, when examined by the microscope, the appearance of reflecting crystals.

It is probable that light exercises a two-fold action on the iodide of silver, whether it is combined or not with chlorine or bromine. By one, the iodide is decomposed, and the silver set free is precipitated on the surface in the form of a white powder or small crystals ; by the other, which begins long before the former, the parts affected by light have been endowed with an affinity for mercurial vapour.

By means of his photographometer, this investigator has been able to ascertain that the pure light of the sun performs in about two or three seconds the decomposition of the bromo-ioclide of silver, which is manifested by the white precipitate; while the same intensity of light determines the affinity for mercurial vapour in the short space of about 1/1000 th part of a second. So that the affinity for mercury is imparted by an intensity of light 3000 times less than that which produces the decomposition manifested by the white precipitate.

For this reason it is difficult to suppose that the two actions are the same. We must admit that they are different. Long before it can effect the decomposition of the surface, light imparts to the sensitive coating the affinity for mercurial vapour; and this appears to be the principle of the formation of the image in the daguerreotype process.

In a paper communicated to the Royal Society on the 17 th of June, 1847, M. Claudet stated that the red, orange, and yellow rays were destroying the action of white light, and that the surface was recovering its former sensitiveness or unaffected state after having been submitted to the action of these rays. It was inferred from that curious fact that light could not have decomposed the surface; for if it had, it would be difficult to understand how the red, orange, or yellow rays could combine again, one with another, elements so volatile as bromine and iodine, after they had been once separated from the silver. These experiments have much in common with those of M. Edmond Becquerel, who has been led to a division of the spectrum into exciting rays and continuating rays. But he had not yet been able to ascertain that, when light has decomposed the bromo-iodide of silver, the red, orange, or yellow rays cannot restore the surface to its former state. The action of light, which can be destroyed by the red, orange, or yellow rays, does not determine the decomposition, which would require an intensity 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, communicate to the surface the affinity for mercury ; and the red, orange, or yellow rays withdraw it. This is in effect the same phenomenon 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 which the red, orange, or yellow rays exercise their destructive action as in the case of the bromo-iodide.