228 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
Some parts of Jupiter, as, for instance, on Jan. 30, 1860, a bright region in its 
northern hemisphere, about 25° in latitude from its equator, afforded traces of chemi- 
cal action in less than a second; the parts of the full Moon, similarly situated rela- 
tively to the Sun and to the Earth, in about the same time. With a similar reflecting 
surface, the illumination of the two bodies should be in the inverse ratio of the 
squares of their distances from the Sun, or as 
125.208" = 1: 27, 
instead of which we have, from the above data: — 
o 8. 2 
Phase of Moon at v = 90 . Ratio of exposures 30 : ipi 
ei vi v — 180. s 4 5:15 = 1:8 to 1:1 for bright belts. 
The disproportion at the different phases of the Moon will be more fully considered 
in another place. The two bodies are presented to us under nearly similar circum- 
stances, as respects the angle of the illuminating rays and the line of vision, at about 
the times of the Moon’s opposition, in which position the ratio of their chemical 
powers, supposing each to be so placed as to receive equal illumination from the Sun, 
would become, — xm 
Photographic intensity of Jupiter — ; x4: 3 times that of the average surface of the full Moon, 
1 | 
“ oe « => NI = 27 « D EE « « 
according as we compare the whole visible surface of the two spheres, or only the cen- 
tral parts of the Moon presented nearly at right angles to the direction of vision and 
of illumination, with the brighter belts of the planet which are similarly situated. 
There is one circumstance which makes the contrast the more remarkable; it is that 
a very small image, like that of the planet, photographs to disadvantage compared 
with a larger one of equal intensity, for want, apparently, of the sympathy induced 
by the chemical action going on in other parts of the plate. I should anticipate 
from this circumstance, that, in a comparison with the Moon, we should be likely to 
underrate the actinic power of Jupiter. 
The explanation suggested by Mr. De la Rue in the passage above duo, attribut- 
ing the differences in photographic power to a diversity in color, cannot be sustained, 
since there is no-satisfactory evidence of any decided difference between J upiter and 
the Moon in point of color, but rather the contrary. A decided blue or green tint in 
Jupiter is only noticed when it is almost in contact with the limb of the Moon. "The 
retina being then excited by the stronger light of the Moon, the planet perhaps 
