CELESTIAL SPECTROSCOPY. if 
when, in 1839, Arago, announcing to the Académiede Sciences the great 
discovery of Niepce and Daguerre, spoke of the possibility of taking 
pictures of the sun and moon by the new process, yet that it is only 
within a few years that notable advances in astronomical methods and 
discovery have been made by its aid. 
The explanation is to be found in the comparative unsuitability of 
the earlier photographic methods for use in the observatory. In jus- 
tice to the earlier workers in astronomical photography, among whom 
3ond, De la Rue, J. W. Draper, Rutherfurd, Gould, hold a foremost 
place, it is needful to state clearly that the recent great successes in 
astronomical photography are not due to greater skill, nor, to any great 
extent, to superior instruments, but to the very great advantages which 
the modern gelatin dry plate possesses for use in the observatory over 
the methods of Daguerre, and even over the wet collodion film on glass, 
which, though a great advance on the silver plate, went but a little 
way towards putting into the hands of the astronomer a photograhie 
surtace adapted fully to his wants. 
The modern silver-bromide gelatine plate, except for its grained tex- 
ture, meets theneeds of theastronomer atall points. Itpossessesextreme 
sensitiveness; it is always ready for use; it can be placed in any posi- 
tion; it can be exposed for hours; lastly, it does not need immediate 
development, and for this reason can be exposed again to the same 
object on succeeding nights, so as to make up by several installments, 
as the weather may permit, the total time of exposure which is 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 can not by the force of con- 
tinued 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 falls wpon the plate is not lost, but is taken in and stored up 
continuously. Each hour the plate gathers up 3,600 times the light- 
energy which it received during the first second. “It is by this power 
of accumulation that the photographic plate may 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 obser- 
vation of faint objects. 
Two principal directions may be pointed out in which photography 
is of great service to the astronomer. It enables him within the com- 
paratively short time of a single exposure to secure permanently with 
great exactness the relative positions of hundreds or even of thousands 
of stars, or the minute features of nebulie or other objects, or the phe- 
nomena of a passing eclipse, a task which by means of the eye and 
hand could only be accomplished, if done at all, after a very great ex- 
penditure of time and labor. Photography puts it in the power of the 
H. Mis. 334, pt. 1 
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