a. _The presence of free Nitrate of Silver essential to the development._--This subject has already been mentioned (p. 36). A sensitive Collodion plate, carefully washed in distilled water, is still capable of receiving the radiant impression in the Camera, but it does not admit of development until it has been redipped in the Bath, or treated with a reducing agent to which Nitrate of Silver has been added: and if the proportion of free Nitrate of Silver on a Collodion film be too small, the image will be feeble or altogether imperfect in parts, with patches of green or blue, due to deficient reduction. b. _Comparative strength of deducing Agents._--No increase of power in a developer will suffice to bring out a perfect image on an under-exposed plate, or upon a film containing too little Nitrate of Silver. But there is considerable difference in the length of time which the various developers require to act. Gallic Acid is the most feeble, and Pyrogallic Acid the strongest, producing at least four times more effect than an equal weight of the crystallized Protosulphate of Iron, and twenty times more than the Protonitrate of Iron. c. _The effect of free Acid upon the development._--Acids tend to retard the reduction of the image as well as to diminish the sensibility of the film to light. Nitric Acid especially does so, from its powerful oxidizing and solvent properties. The effect of Nitric Acid is particularly seen when the film of Iodide of Silver is very blue and transparent, and the quantity of Nitrate of Silver retained upon its surface small. Under such circumstances the proper development of the image may be suspended, and spangles of metallic Silver separate. This indicates that the quantity of the acid should be diminished, or the strength of the Nitrate Bath and of the reducing agent be increased, as a counterpoise to the retarding action of acid upon the development. Acetic Acid also moderates the rapidity of development, but it has not that tendency altogether to suspend it possessed by Nitric Acid. It is therefore usefully employed, to enable the operator to cover the plate evenly with liquid before the development commences, and to preserve the white parts of the impression from any accidental deposit of metallic Silver due to irregular action of the reducing agent. On comparing the retarding effects of free acid upon the light's action, and upon the development, we see that the former is the most marked,--that a small quantity of Nitric Acid produces a more decided influence upon the impression of the image in the Camera than upon the bringing out of that image by means of a developer.