The manipulations and chemicals employed in the production of the negative collodion pictures are very similar to those already given for operating by the positive process; frequent reference will therefore necessarily be made to portions of that process, as described in the preceding pages, and only such parts will be given here, as do not correspond with the foregoing. It is thought advisable to omit in this chapter every reference that does not have a desired tendency to aid the operator in the plain straightforward order of manipulation. The negative process is fast becoming popular and needs the attention of all who desire to keep pace with the experiments in the art. Since the first edition of this work it has been my pleasure to see many fine photographic specimens produced by the following process, and no one need fail, if he will carefully adhere to the details given. There perhaps may be circumstances making it advisable for some to have but one nitrate of silver solution for both positive and negative collodion pictures: for such, a process will be given in the following pages, which has recently appeared in _Humphrey's Journal_, and is called, after its author, the "Helio Process," this is well adapted for most purposes. Soluble Cotton. The method for preparing this has been given in page 41. It is prepared in the same manner for both positives and negatives. Plain Collodion. The preparation of plain collodion employed is the same as that described at page 53. Developing Solution for Negatives. Rain or distilled water 6 ounces. Protosulphate of iron 300 grains. Acetic acid 2 ounces. A little alcohol may be added to make it flow more evenly over the plate--say 1 oz. This solution can be kept in a pint bottle, and should have a funnel devoted solely to the purpose of filtering it. One of the most convenient dishes for receiving this solution, when poured over the plate, is a bowl with a lip to it, as it can be readily poured back into the funnel. The mode of employing this developer is the same as that for positives, described at page 133. It may be used an indefinite number of times, but should be kept clean; it soon assumes a red color. Re-developing Solution. This solution is for the purpose of giving increased intensity to the negative, but as its use in the hands of beginners is attended with some difficulty, I would not recommend the operator to try it until he has had considerable experience in the developing process, or he will undoubtedly spoil his proofs. Its use requires promptness of action and quick observation. The following is the formula for its preparation: Water 4 ounces. Protosulphite of iron 400 grains.