The constant variation of the light renders it impossible to lay down any exact rule upon this point.

Light is not alone to be considered; the amount of coating exercises a deviating influence, also the subjects to be represented are not equally photogenic, some requiring much longer time of exposure than others.

This may be easily observed by exposing the plate at the same time to a plaster bust and a piece of black velvet, the first being a much stronger reflector of light than the latter: the time necessary to produce a well developed image of the velvet being about six times longer than that required to produce an equally defined image of plaster.

The manner of judging correctly of the time is by the appearance of impression after it has been developed by the mercurial vapors.

Should it present a deep blue or black appearance it is solarized or over-timed.