ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 223 
multiply the number of individual determinations, and to vary the methods of obser- 
vation, so as to approach the subject from as many different quarters as possible. 
Even when this has been done, it is not always that we can be secure of assuming 
correct principles on which to proceed in deducing from the data their legitimate 
consequences. 
The times of exposure required to produce daguerrotypes, or strong “negative” 
photographs, of the Moon and Jupiter, have been derived from observations made upon 
twenty-one nights from 1851 to 1860 inclusive. 
On classifying those for the Moon according to its angle of elongation from the Sun, 
it appears that there is a continuous decrease in the duration of exposure from new to 
full moon. At the latter phase, when the elongation is 180°, only one sixth as much 
time is required as at the phase for which the elongation is 60°, the Moon then being 
four or five days old. | 
The question, it must be remembered, is one of intensity, not quantity of light. In 
the latter the disproportion is very much greater, because the difference in the extent 
of illuminated area is also to be considered. The above results have suggested an 
examination of the proportions between the illuminating power of moonlight at the 
several phases, which are considered in another connection. 
By comparing the exposures for the Moon with those for Jupiter, the photographic 
intensity of the latter is found to be nine times that of the average surface of the full 
Moon, including the whole visible hemispheres of both bodies, and twenty-seven times 
greater if we have regard only to the central regions of the Moon and the bright belts 
of the planet. From a subsequent discussion of all the data, it was concluded that 
Jupiter reflects out of a given quantity of incident light, fourteen times more of the 
chemical rays than the Moon does. In other words, that the latter, if the constitution 
of its surface resembled that of Jupiter, would photograph in one fourteenth of the 
time which it actually requires; but if we have regard only to the brightest regions of 
the planet, the disparity in times of exposure must be increased to the ratio of perhaps 
1 to 30, or even more. 
It is shown that there is not sufficient difference in the color of the two objects, to 
“account for this inequality in photographic power. This is proved by comparisons 
with artificial light of different tints, reflected from silvered glass globes side by side 
with the images of the Moon and Jupiter, and by other evidence. The artificial illu- 
mination from the “ Bengola” or “blue-light” nearly resembled moonlight in color. 
The distribution of light over the discs of Jupiter and the Moon presented, in the 
photographs, a decided contrast; the former is brightest near the centre in. zones par- 
allel to the equator, the latter near the margin. 
