"Before beginning to work, be sure that your plate is clean. Tin plates, with which you would better work until you become skillful in handling the baths, are sold in sheets by any dealer in photographic goods, and when you buy them are often covered with fine dust. Polish them well with a pad of soft chamois-skin before you proceed farther. Next pour the collodion on the centre, and cause it to flow evenly by gently tipping the plate from side to side. Allow the surplus to drip off into a flask; and as collodion is an expensive article, you would better mix some gum-arabic and water to about the required thickness, and practice with that first, that you may not waste the more costly fluid by failures to spread it evenly on the plate—a very difficult matter for beginners to accomplish. This collodion is made of alcohol, ether, and gun-cotton, and sensitized with certain iodides and bromides. It evaporates if exposed to the air, and must be well corked, and kept in a cool, dark place, as both lights and heat are injurious to it. A positive collodion is often sold for ferrotypes, but the negative fluid gives better results.
"When the film of collodion has become set, the plate is ready for the sensitive bath. Place it on a strip of glass bent at the lower end, which you will buy with your bath dish, and lower it into the bath quickly; otherwise a line may be noticed on the finished picture, due to the uneven deposit of silver. The deposit may be hastened by gently moving the plate in the liquid. After a few seconds lift it out and examine it. If it is streaked and greasy, it must be put back; but when it is of a fine opaline tint, free from streaks and flaws, it is ready to be placed in the camera, which should be already properly focussed and in position.
"Now, boys, comes the great trouble—to correctly time the exposure. It varies from five to forty-five or sixty seconds, according to the light, the arrangement of your screens, and the condition of the silver bath.
THE FIRST ATTEMPT—SOMETHING WRONG.
"When you think, from the nature of the case, that your plate has been exposed long enough, close your slide, and return to the dark room, where you now proceed to develop your picture. You must have already mixed this developing solution: one fluid part of sulphate of iron, one and a half fluid parts of acetic acid, and sixteen parts of rain-water. Do not make too much of this at once, as it quickly becomes spoiled. When you take the plate from the slide, you will see no alteration in it, but when you pour on some of your developer, 'as if by magic a picture appears.' See that the developer flows all over the plate, and do not allow it to settle on any one place, as this would make a stain which can not be removed.
"As soon as the development is complete, wash the plate well with pure water, using for the purpose a wash bottle, which is simply a large glass flask having a cork perforated by two tubes, one of which reaches into the body of the liquid, while the other only passes through the cork. The short tube is bent over at an angle so that the mouth may be conveniently placed against it, while the long tube is bent, and drawn out to a fine jet. On blowing through the short tube, the air in the bottle becomes compressed, and in expanding drives the liquid through the jet in a fine steady stream. When the plate has been well washed, it must be treated with another solution, as this picture is one that would soon fade, just as you no doubt have seen proofs of photographs do. To remove the unaltered silver a solution of hyposulphite of soda in water is used. Cyanide of potassium is also used, because it is much cleaner.
"But there is no rose without its thorns, and the cyanide makes up for its cleanliness by being one of the most deadly poisons, and I would advise boys who are not posted on the fine points of chemical manipulation to have nothing to do with it. This fixing solution is made of eight ounces of the hyposulphite and forty of water. Now if this is made too strong, it will spoil the picture, so it is well to be careful to have the exact proportions.
"By-the-way," added the Professor, "if you do use cyanide of potassium, be very careful not to get any of it into what cuts or bruises you may have on your hands. Boys always have such ornaments, and if the cyanide touches a place where the skin is broken, it is liable to mix with the blood, and make trouble.
"After your picture is fixed, wash it well and varnish it. Ten parts of gum-arabic to one hundred parts of water will make a very fair varnish; but as this has to be dried over a spirit-lamp, it is better to buy the self-drying varnish which is sold for this purpose.