tests of chloride of silver, among chemists, is that it shall darken in the light. Here I have a little bulb of it which was prepared, dried carefully, and sealed up. It has been exposed for months to the light, and is as pure a white as it was the first day it was put into the bulb. Another experiment was made at the same time; but, unfortunately, as I thought then, a small globule of mercury got into the vacuum, and was sealed up with the chloride; the consequence was that the chloride of silver immediately darkened: although the mercury was not in contact with the salt, the chlorine flew to the mercury, and formed chloride of mercury. This is an instructive experiment, showing that chloride of silver will darken merely in the presence of something that will mop up the chlorine. Silver iodide, when exposed to light, splits up into silver subiodide and iodine, and silver bromide into silver subbromide and bromine. Now, in order that there shall be a ready darkening of either of these salts, you must have something which will absorb the iodine or bromine (or, in the case of the latter, allow it to escape), according to the salt you expose to the light. This something is the sensitizer.
One point that has exercised the minds of a great many photographers is the illumination of their dark rooms. [The lecturer having shown the relation of the several parts of the solar spectrum with the absorption properties of different substances used in photography, proceeded to demonstrate the effect of differently colored glasses upon the passage of rays, and announced his conclusions.] If photographers want to have an absolutely safe light in developing their pictures, let them glaze their studiosglass and stained red, and they will get nothing but the light of that particular refrangibility, which will not affect any gelatine plate of the ordinary type. You may glaze and glaze with ruby, but you will never get rid of blue light entirely. Of course, it diminishes with every thickness you take. If you want to use ordinary plates, which are not so sensitive that you can not look at them, my advice is to use a combination of stained red and ruby glass, which will give you a comfortable light to work in, for it cuts off the blue and leaves the red in a brilliant patch. If the operator wishes to be still more secure, let him use a combination of cobalt glass and stained-red glass. A combination of red and green is a fairly safe light for iodide plates or ordinary plates, but not for gelatine plates, which are very sensitive. Next we come to a series of pretty colors, which may be very useful to us: magenta, with which the yellow is cut out entirely, and the green, leaving the blue, violet, and orange; aurine and chrysoidine, which cut off the blue; a combination of magenta and aurine, which gives a perfect red light, and is very good indeed for the photographic studio; and scarlet and aurine, which give the same effect. If all means of securing the right light fail, the photographer may use the ferrous oxalate developer, for you may bring the most sensitive plate out into a white