of the negative in the camera by the collodion process, Dr. Vogel gives the following directions:
While preparation is being made to take the picture, expose a plate to the light at the place where the model is to sit, one minute, timing it with the watch. Now expose the photometer for one minute in the same light, and note the degree indicated on the sensitized slip.
We believe this instrument will be found useful to every photographer, and, as we understand, it will be furnished at a low price; no doubt all who desire to progress and improve will possess themselves of one or more. Keep the glass clean above the scale. Don't touch the scale with damp fingers.
THE CHROMIC SALTS.
Either chromic acid alone, or various of its salts, may be used to render certain soluble organic bodies insoluble after exposure to the action of light. Practically, for a variety of reasons, the bichromate of potash, or the bichromate of ammonia, is found to answer best. Bichromate of potash, as being the cheapest, is more generally employed, otherwise bichromate of ammonia has certain advantages. It is a little more sensitive, and has been said to yield a tissue less liable to spontaneous insolubility. The latter quality, however, seems doubtful, and certainly requires verifying; as, for more than one reason, it seems probable that it will, on the contrary, produce a tissue more prone to spontaneous decomposition. In the first place, this tendency will necessarily accompany extreme sensitiveness. Mr. Swan finds that dampness in the sensitive tissue is a chief cause of the spontaneous change which produces insolubility, and as bichromate of ammonia is more greedy of water than bichromate of potash, the tissue will more
