Mix the water and acetic acid first; then dissolve the iron; and, lastly, add the nitric acid, which, by varying the quantity, produces different effects. On pouring the solution over the plate, there is sometimes a difficulty experienced in causing it to flow evenly. Sometimes a little more acetic acid in the developing solution, or, if the plate has been out of the bath some time, redipping it, will prevent this; but if this does not remove it, and the resulting picture is hard and unpleasant in tone, a new bath is necessary. For positives, the resulting picture is more pleasing and delicate by using the developing agent rather weak. After it has remained on sufficiently long to bring out the image, the undecomposed iodide is to be removed by hyposulphite of soda. I always use the same solution, pouring it on and off until exhausted. Having sufficiently washed, the picture may perhaps appear with many black spots, this may in future be obviated by adding a little alcohol to the collodion:--or it may be covered with white spots; in that case the collodion requires settling, or rapidly filtrating through an old piece of loose silk. Sometimes it will look all black and white (a common fault with collodion positives), without middle tints: by adding a little more acetic acid, or an extra drop of nitric acid, to the developing solution, or the addition of a few drops of ordinary pyrogallic solution, this disagreeable effect may be overcome. In taking portraits, it is often caused by having the sitter placed with too much front light. Then, again, the should-be whites of the picture may be dull and greenish by reflected, and red by transmitted, light. This effect I generally find remedied by putting less nitric acid in the developing solution. During the development, by watching the colour (by holding a piece of white paper underneath), this red tendency may be observed; in that case the drawing may be preserved by leaving the plate for about a minute after pouring the developing agent off, and before removing the iodide. Some change appears to take place by its contact with the air; it gradually gets more opaque, and when finished, though not so white as many, yet presenting an extremely rich brownish-yellow tone.