299 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
appear to be not greater than as from 6 to 5 or 6 to 4. It is not improbable that the 
blue tint of Jupiter may have something to do with its photogenic power. It may be 
also stated, that the darkest parts of Jupiter’s surface came fully out by an exposure 
which did not suffice to bring out those portions of the Moon situated near the dark 
limb, and consequently illuminated by a very oblique ray; thus confirming an obser- 
vation already communicated by me." 
* As the night advanced, I was able to take pictures of Jupiter in five seconds, in 
consequence of the planet attaining a greater altitude; and the position of Saturn 
being favorable for a comparison of its actinic power with that of Jupiter, I turned the 
telescope alternately on each of these two planets, and found that, to produce pictures 
of equal intensity, the sensitized plate had, on the average, to be exposed 5 seconds 
to Jupiter, and 60 seconds to Saturn. Hence the chemical rays from Jupiter are 
twelve times more energetic than those from Saturn, — an effect undoubtedly in a great 
measure attributable to the greater brilliancy of the former planet, but not, I believe, 
entirely so.” * 
Further comparisons between the Moon and Jupiter, made at the Observatory of 
Harvard College in 1860, have suggested a variety of experiments, relating to their 
optical as well as to their photographic intensities, which will be described in the fol- 
lowing pages. A short explanation of the methods used in the reduction of the 
observations, and a general summary of the conclusions to which they have led, will 
first be given. 
The latter must be received with proper allowance for the difficulties incident to all 
photometric experiments, and perhaps not less so to their subsequent interpretation. 
We have no means of measuring degrees of optical intensity, other than by the pre- 
carious standard afforded in the sensation produced upon the eye, and since this 
involves much that is merely subjective and liable to alter with each new observer, 
or even with the same individual at different times, the data must be exposed to con- 
siderable uncertainty from this source alone. The photographs, it is true, are not 
affected in the same way, yet they have their peculiar failing in the irregular action 
of the chemicals. Again, the variable clearness of the sky, and differences of color 
and quality of light, have an influence both upon the photographic and optical rays, 
which it is impossible to eliminate completely. For these reasons, it is desirable to 
* Monthly Notices Royal Astr. Soc., Vol. XVIII. p. 55. The instrument was a Newtonian reflector of 
13 inches’ aperture. De la Rue photographed Saturn side by side with the Moon, May 8th, 1859, and 
obtained a distinct, though somewhat faint picture, in 15°. The intensity of sunlight at this planet is only 
about 4! of that at the Moon. - e 
LJ 
