PHOTOGRAPHIC PLATES. 
103 
If it is seen on development that some mistake has 
been made in exposure, it is, as a rule, of little use to 
alter the exposure slightly; it is better to double or 
halve it. Exposures should be varied in geometrical 
rather than in arithmetical “ progression ” ; if an ex¬ 
posure of 40 seconds is found to be wrong it is better to 
try 20 or 80, or even 10 or 100 rather than 35 or 50. 
For some reason we find that if we get a poor negative 
the remedy, in go per cent, of the cases, is to increase 
the exposure. 
If the background of a negative is weak we have to 
look to one of three causes : 1st, under-exposure ; 2nd, 
reflected light inside the apparatus ; 3rd, gross over¬ 
exposure, but in this case the whole image is weak and 
grey. Under this head may fall the effect of flooding 
the lens with too much aperture, but this is seen in the 
microscope; and under-development, which is also easy 
to discover and to avoid in future. 
