It is very singular that the picture can be alternately changed from a white positive to a black negative many times in succession, and very often with improvement.
Thus, by the above process, a most perfect white positive or a deep black negative is produced, quite distinct from each other.
In the first part of this after-process, it will be observed that the effect of this bi-chloride of mercury solution is to deepen the shades of the picture, and this peculiarity can be made available to strengthen a faint image, by taking the precaution of using the solution weaker, in order that the first change may be completed before the whitening effect comes on.
The progress of the change can be stopped at this point by the simple appliciation of water.
M. Adolphe Martin has published some remarks on the collodion in the Comptes Rendus of 5th July, 1852.
The collodion he employs is made of 30 grains of cotton.
750 grains of nitrate of potash. 1500 grains of sulphuric acid.
This is well washed and dissolved in 10 volumes of ether and 1 volume of alcohol : by this, 15 grains of gun-cotton are dissolved in 1860 grains of ether, and 930 grains of alcohol : add then to this collodion, 15 grains of nitrate of silver transformed into iodide, and dissolved in 20 grains of alcohol by means of an alkaline iodide. M. Adolphe Martin prefers iodide of ammonium. The plate is next plunged into a bath of 1 part distilled water, 1 1/12 th nitrate of silver, and 1 1/20 th nitric acid. The image is developed by proto-sulphate of iron, and he effects the change from negative to positive by a bath of double cyanide of silver and potash, consisting of about 2 quarts of water, in which dissolved 375 grains of cyanide of potassium, and 60 grains of nitrate of silver. The pictures thus produced are remarkable for their intense whiteness.
We must allow Mr. Archer to give his own description of the very ingeniously constructed camera, which he has devised for the collodion process. The practice of the collodion process in the open air is a matter of some difficulty ; some ingenious cameras have been devised for this purpose; but there are few better contrivances than that introduced by Mr. Archer, and modified by Messrs. Griffin & Co.
" I will proceed to give a general description of the camera I have constructed, premising that it admits of being made as a very light folding camera, if thought necessary.
"It is a wooden box, 18 inches long, 12 inches wide, and 12 inches deep, and is capable of taking a picture 10 inches square. Externally it may be thus described :—In front it has a sliding door, with a circular opening in it, to admit the lens : this sliding door enables the operator to lower, or raise, the lens, and consequently the image formed by it, on the ground glass, as the view may require. The two sides of the camera have openings cut in them, into which sleeves of India rubber cloth are fixed, to admit the hands of the operator, and are furnished with India rubber bands at the lower ends, which press against the wrists, and prevent the admission of light.
" The back of the camera has a hinged door fitted at its upper part with an opening of just sufficient size for the eyes, and shaped so as to fit close to the face. A black cloth is tied round this end of the camera, to prevent any ray of light penetrating at this opening. In the top of the camera, near the front, is inserted a piece of yellow glass, to admit a small quantity of yellow light, and is closed with a hinged door, to regulate the quantity of light required.
" The interior of the box is furnished with a sliding frame, to support the ground glass or the bath and the prepared plate ; and it has a stop, by means of which any focus from 3 inches to 15 inches can easily be obtained.
" The bottom of the camera is furnished with a gutta percha tray, about 1 inch deep, to hold the washings, etc., when the camera is in operation.
" Also, the bottom of the camera at the back has an opening cut in it, extending nearly the whole width of the camera, and as far in as the edge of the gutta percha tray.
" This opening is intended to admit, when the camera is in use, a light wooden case containing the glass bath, focusing frame, stock of glass, and paper required in the process.
"There are various other little contrivances which I have not specified; such as a drawer for the pictures, a shelf for bottles, etc.
This form of camera will admit of the following manipulation :—Having placed it upon a stand pointing to the object to be taken, the hinged door at the back is opened, and the bath is three parts filled with the solution of nitrate of silver ; a plate of glass is then taken from the cell, and cleaned if necessary.
"The collodion is poured on in the manner previously described ; when the film has set a little it is immersed in the nitrate of silver bath, and the lid of the bath is closed down upon it. The next step is to obtain the focus with the ground glass : this can be done whilst the collodion is becoming iodized.
"After adjusting the sliding frame to the proper focal distance, the camera must be closed, and the rest of the process conducted by passing the hands through the sleeves, and placing the eyes close to the aperture in the back of the camera, and drawing the black cloth over the front of the head.
" By the aid of the yellow light admitted from the top, the operator can carry on the rest of the process. The plate is now ready for the action of light, and is taken from the bath; or the bath itself, with the plate in it, is placed in the sliding frame The refracted image is at once thrown upon the sensitive plate. After the requisite exposure, the plate is taken from the bath, and the picture is developed with the solution previously described. The progress of this operation can be seen by aid of the yellow light, keeping the eyes close to the aperture behind.