Perhaps the simplest advice we can give as to the particular developer to be used is to take as the normal developer one mixed according to the formula sent out with the plates which the student has chosen to work with, but the student must not use it in the exact proportions given by the maker. Let the student mix up the stock solutions as told, varying the constituents as the case in hand demands. If he has carefully and thoroughly read his chemistry, and if he remembers the hints we have given him, he will have no difficulty in following out the directions.

He should, as a rule, never use more than two-thirds of the amount of pyrogallic acid recommended; let him be very careful how he uses the restrainer, and let him add the ammonia only in small quantities, unless the exposure has been very rapid. As a rule let him work with weak developers. We could easily give a dozen or even fifty formulae for developers, but the student would be no wiser if we did, only more confused. Every photographer fancies his own particular formula, but we have no belief in any special favourites; we have worked with many, and find the results depend altogether on the quantities used and the manner of developing rather than on the constituents. Take, then, the formula recommended by your plate-maker, but use it, as we have said, with judgment. Begin with a sufficiency of pyrogallic acid (according to the subject), use little restrainer, except in over-exposure, and add the ammonia slowly, adding a few drops from time to time as required. In short, make, it your rule to use weak developers, and develop slowly. If you think you are likely to have under-exposed, add ten to twenty drops of a one per cent, solution of hyposulphite of soda, using no restrainer. Some unscientific persons imagine that development can be reduced to a science, and that absolute quantities of each solution must be used. One might as well expect a physician always to prescribe the same doses. Each picture requires a developer of its own; that should never be forgotten. We have tried hydrokinone instead of pyrogallic acid; a given quantity of hydrokinone does the work of double that quantity of pyrogallic acid, but it has no advantages, so far as we can see, except for the development of under-exposed plates. For very rapid work we recommend the carbonate-of-potash developer, as green fog does not result. The formula we use is Dr. Eder's:-

Eder's Potash Developer