A Manual Of Photography

240 PRACTICE OF PHOTOGRAPHY. quired for polishing them a substance which does not adhere to the silver ; pumice, which I recommended at the commencement, leaves less residue. "As regards the liquid to be employed : in the first operations nitric acid of five degrees must be employed, as I stated in the first instance ; but for the last operations it must be reduced to one degree. “ The polishing with oil and the heating may be suppressed. “ I take the opportunity afforded by this communication to lay before the Academy the following observations, which I owe to experience :— “ The layer produced by the descending vapours of the iodine and of the accelerating substances forms with silver a more sen- sible compound than is obtained with the ascending vapours. I make this observation only to lay down a fact, for it would be difficult to employ descending vapours, on account of the dust which might fall during the operation, and from stains. “ The resistance which light experiences in passing through a white glass is well known. This resistance is even greater than it appears, and may be attributed not only to the dust which is left on the glazing in cleaning it, but also to that which is naturally deposited on it. The object-glass of the camera obscura is certainly in the same case. To ascertain this, I put the object-glass in cold water, which I boiled ; I knew that it was impossible to remove it without the sides. This operation had, therefore, no other object than to raise the tem- perature of the glass to 212° F. C., and I then immediately poured on the two sides of the object-glass very pure boiling water to remove the dust. By operating directly with the object-glass, thus cleansed, I still further increased the prompti- tude. This means presents too many difficulties to be put in practice ; only care should be taken to clean the object-glass every day?....,. " The atmospheric dust, which is the scourge of the photo- genic images, is, on the contrary, favourable to images which are obtained by contact or at a very short distance. lo be con- vinced of this, we have only to clean the two bodies which we wish to put in contact with the boiling water, as I have just in- dicated, and to keep them both at the same temperature as the air ; there will then be no impression, which evidently proves that these images have no relation with the radiation which gives photographic images.” 1 Professor Stokes has recently confirmed the truth of this by some very conclusive experiments.