By Sensitiveness is meant a facility of receiving impression from very feeble rays of light, or of receiving it quickly from brighter rays.
Intensity, on the other hand, relates to the appearance of the finished Photograph, independently of the time taken to produce it,—to the degree of opacity of the image, and the extent to which it obstructs transmitted light.
It will be seen as we proceed that the conditions necessary to obtain extreme sensitiveness of the Iodide film are different from, and often opposed to, those which give the maximum intensity of image.
CAUSES WHICH INFLUENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION.
Some of the most important are as follows:—
a. The presence of free Nitrate of Silver.—When the sensitive film is removed from the Nitrate Bath, the Iodide of Silver is left in contact with excess of Nitrate of Silver. The presence of this compound is not essential to the action of the light, since, if it be removed by washing in distilled water, the image may still be impressed. In such a case however the effect is produced slowly, and a longer exposure in the Camera is required.
The sensitiveness of the Iodide film does not increase uniformly with the amount of the excess of Nitrate of Silver, as measured by the strength of the Bath. It is found that no advantage in this respect can be gained by using a proportion of Nitrate of Silver greater than 30 or 35 grains to the ounce of water, although solutions of three times this strength have been sometimes employed.
It has been asserted that a chemically pure Iodide of Silver, which is unaffected in colour by the direct action of light, is also incapable of receiving the invisible image in the Camera; and that the sensitiveness of a washed Collodion film is due to a minute quantity of Nitrate of Silver still remaining. Iodide of Silver in the state in which it is thrown down on diluting with water a strong solution of the salt known as the double Iodide of Potassium and Silver,—and which must, from the mode of its preparation, be free from Nitrate of Silver,—is quite insensitive; but this form of Iodide differs from the other in colour, and not only so, but is likely to contain an excess of Iodide of Potassium. The application of a solution of Nitrate of Silver to this compound at once renders it sensitive to light.
b. Free acids in the Nitrate Bath.—Strong oxidizing agents, such as Nitric Acid, greatly diminish the sensibility of the film, and hence the importance of removing the free acid often met with in commercial samples of the Nitrate of Silver. The effect of even a single drop of strong Nitric Acid in an eight-ounce Nitrate Bath will be appreciable; and when the proportion is increased to one drop per ounce, it will be difficult to obtain a rapid impression.
Acetic Acid has far less effect upon the sensitiveness than Nitric Acid, and being found useful during the development of the image is commonly employed; but when great rapidity is desired, it should be added cautiously, and in a proportion very much less than that in the solution known as the Aceto-Nitrate of Silver, which contains about one drop of the glacial acid to each grain of Nitrate of Silver.