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Good photographs of the sun were, however, taken much 
earlier, the conditions of the problem being simpler. Even as 
soon as 1845 Fizeau and Foucault got a fine Daguerreotype of 
it. Much of our recent knowledge of solar physics is a direct 
outcome of the application of photography to the solution of the 
great problems which present themselves more prominently 
during an eclipse. In 1863 Huggins had already photographed 
the spectra, but it was in 1868 that the spectroscopy of the 
eclipsed sun virtually began. It was in this year, too, that the 
first notable results were obtained by photography of the height 
and appearances of the protuberances and of the extent of the 
corona. 
The aspect of the planets also has not been overlooked, but 
owing to the faint light which they emit, their great distance, and 
the slight variation in tint of the markings on their surfaces, the 
results are not so striking or valuable as those obtained from the 
sun and moon, 
In 1881 the perfection of the delicate and sensitive gelatino- 
bromide plate, placed a new instrument for research in the 
hands of the astronomer. The waves of light, too faint to affect 
the retina of the eye even when concentrated by a large telescope, 
accumulate their effect in a long exposure of a gelatino-bromide 
or chloride film to their rays. The continued ripple of vibrations 
are competent to bring about those molicular changes in the 
unstable equilibrium of the silver salts, and impressions are thus 
registered of appearances beyond the utmost limit of human 
vision, even when aided by the largest telescopes. The magni- 
ficent photograph of the nebula in Orion obtained by Draper 
in 1881, with an exposure of one hour, and the still later achieve- 
ments of Common and Roberts with exposures of three and four 
hours, reckon among the most astonishing and suggestive results 
achieved in the domain of celestial photography. 
But photography was destined to throw a light in a manner 
little suspected at first on the great problem of the motions of 
the fixed stars. Doppler, basing his reasoning on the analogy 
between light and sound, enunciated the theory in 1842 that the 
accession or retreat from our system of a star would reveal itself 
- in a shifting of the lines of the spectrum, and that the variation 
from anormal would afford us the means of measuring the vast 
orbits which they traversed in these abysses of space beyond our 
tiny system. In 1868 Huggins confirmed by direct observation 
the truth of Doppler’s speculation, and showed by comparison of 
the spectrum of the same star, taken at different intervals, that 
