We have had these appear in a few negatives some months after development. We know of no remedy for the defect; nor do we know the cause, but believe it to be due to hyposulphite of soda left in the film.
Deposit on film.
This is sometimes met with after the imperfect washing out of hyposulphite of soda; or sometimes whilst the negative is in the fixing bath, if it has been in the alum bath previously, and not thoroughly washed. Sulphur is deposited. The remedy is obvious.
Metallic patches.
Coloured metallic-looking patches appear at times near the edges of the plate, which may, or may not, be accompanied with fog. We have often observed these patches in plates which have been kept a long time. There is no remedy if they are unaccompanied by fog, but if fog is present, the ferric-chloride solution will generally remove them.
Scratches.
On the back of the negative show as dark lines in the film.
Undeveloped islands.
Rarely, we have met with small patches which seem to have refused to develop; they are generally circular. Captain Abney says they are due to the use of chrome alum in the emulsion. There does not appear to be any remedy for this accident.
Dull spots and pits.