Referring to the new and great power which modern photography had put
into the hands of the astronomer, the president said that the modern
silver bromide gelatine plate, except for its grained texture, met his
needs at all points. It possessed extreme sensitiveness, it was always
ready for use, it could be placed in any position, it could be exposed
for hours, lastly it did not need immediate development, and for this
reason could be exposed again to the same object on succeeding nights,
so as to make up by several installments, as the weather might permit,
the total time of exposure which was deemed necessary. Without the
assistance of photography, however greatly the resources of genius
might overcome the optical and mechanical difficulties of constructing
large telescopes, the astronomer would have to depend in the last
resource upon his eye. Now, we could not by the force of continued
looking bring into view an object too feebly luminous to be seen at
the first and keenest moment of vision. But the feeblest light which
fell upon the plate was not lost, but taken in and stored up
continuously. Each hour the plate gathered up 3,600 times the light
energy which it received during the first second. It was by this power
of accumulation that the photographic plate might be said to increase,
almost without limit, though not in separating power, the optical
means at the disposal of the astronomer for the discovery or the
observation of faint objects.