The most successful operator with the bromodized collodion appears to have been M. Laborde, who communicated the following as the results of his practice to La Lumiere, French photographic journal:—
" I have studied the action of bromides by themselves and in combination with collodion. My choice naturally fell upon those soluble in alcohol or ether, and I have tried the bromides of iron, nickel, cadmium, zinc, and mercury. The bromides of iron, nickel, and cadmium yielded the best results, and among these I give the preference to the bromide of cadmium. I have found it to possess so many advantages, that I have been several times tempted to banish all iodides from my preparations. 15 grains of bromide of cadmium, added to 1 1/2 ounce (by weight) of solution of collodion, gives a liquid which may be used at once, and which has been kept for about five months, up to the present time, without perceptible change. Sulphate of iron, or pyrogallic acid, are used for developing; gallic acid produces but a very middling effect; almost all the details of the image appear at once, the effect of the weakest radiations becomes sensible; but the extreme tints of the proof are not sufficiently different to allow of being printed from with success. The following process is made use of to give them an actual value in this respect.
" By adding a weak proportion of iodide of potassium to the bromide of cadmium the sensibility is increased, and we obtain at the same time a greater difference between the extreme tints of the proof; the negatives are therefore superior. The following are the proportions usually employed:—
Bromide of cadmium .... 12 grains. Iodide of potassium . . . .0.3 grain. Collodion . . ... . . .1 1/2 ounce.
"At first the iodide of potassium tinges the collodion with yellowish-red, but the bromide of cadmium by degrees removes this tint, and the solution becomes colourless".
Mr. W. Crookes has lately commenced the investigation of these phenomena with a zeal and accuracy from which we may expect some important results. The following communication, made to the Photographic Society of London, will be read with interest:—
" I have for some time past been working with bromized collodion, and as, from my experience, it seems likely to become an agent of great value, perhaps the following account of some experiments with it may prove of interest to any photographer, who has the time and means at his disposal to investigate the subject more fully in its practical application.
" To prepare the collodion I proceed as follows:—Mix together equal bulks of sulphuric acid, specific gravity 1.80, and nitric acid, specific gravity 1.50; stir well with a glass-rod, and then,while still warm, immerse as many pieces of good Swedish filtering-paper as the vessel will conveniently hold. Allow them to remain together for one hour; then pour the liquid away, and wash the paper until free from the slightest trace of acid, and allow it to dry in a warm room.
" Take 8 drachms of the purest washed ether and 1/2 drachm of spirits of wine, 60° above proof, and dissolve in this 6 grains of the above-prepared paper. This collodion may be bromized in the following manner:—In a small bottle place about 2 grains of crystallized nitrate of silver and about 10 grains of pure bromide of ammonium; pour on this 2 drachms of spirit 60° above proof, and allow them to remain together for some hours, shaking the mixture several times. One drachm and a half of the supernatant liquid are to be added to every ounce of the previously prepared collodion. Thus bromized, it will remain perfectly colourless and good for a long time.
" I excite the plate in a 30-grain silver bath, which has been previously saturated with bromide of silver; about two minutes' stay in this bath is generally sufficient, though a little longer time does not injure it. The film of bromide of silver is a pale orange by transmitted and blue by reflected light, and is very transparent. For developing, I prefer protonitrate of iron, being more accustomed to it, but have no doubt that in other hands pyrogallic acid would answer equally well. The above proportions of paper, alcohol, and bromizing compound may be varied within certain limits without much influencing the result. I have given the proportions which I am most in the habit of using, but would recommend that the experimentalist should ascertain for himself whether a slight departure from the above proportions would give a collodion of the consistency and strength of that with which he is most accustomed to manipulate.
" The chief advantages it seems to possess over the ordinary iodized collodion, besides its great sensitiveness, are the following. In a landscape the required opacity of the more strongly illuminated parts (the sky, for instance) is not lost by overexposure; vegetation is also more easily copied. Its superior sensitiveness to coloured light is, however, most strikingly shown when coloured glass or sulphate of quinine (as suggested by Sir John Herschel) is employed to absorb the strongly acting invisible rays. To prove this, I arranged several flowers and plants with a view to obtain a great contrast of colour, light, and shade. The best picture I could obtain of them on iodized collodion was, as I had anticipated, wanting in half-tint, very few of the colours producing an adequate impression. When, however, bromized collodion was used under the same circumstances, but with the interposition of the bath of sulphate of quinine, every part came out with nearly the same gradation and depth of light and shade as existed in nature : this picture on bromized collodion behind the quinine bath required 40 minutes; on iodized, without the quinine solution, 4 minutes; but when I attempted to take a photograph- on iodized collodion, with the quinine bath interposed, I found that with the light I was working in, the plate would not keep a sufficient length of time to enable me to obtain an image.
" The advantages of using bromine in the place of iodine, for all objects having a green or yellow colour, have been already pointed out".