COMPARISON OF THE LIGHT OF THE SUN AND MOON. 293 
We may connect this result with the comparisons instituted between the Sun and 
the full Moon by the following process. 
Seidel's photometric determination of the relative brightness of the planet Jupiter, 
at its mean opposition, compared with Sirius, gives 
Jupiter at mean opposition 
Log — = 0.2952.* 
Sirius 
We have found in the preceding memoir, 
Jupiter at mean opposition — 1 ` 
Mean Full Moon ~ 6430 ' 
hence, 
Sirius SR 
Mean Full Moon ~ 12688 ' 
Applying this to Wollaston’s ratio between the light of Sirius and the Sun, we 
obtain 
‘Sunlight 
— —— = § = 1576100 
Full Moonlight : 
which is nearly twice as large as the value obtained by the previous method. There 
can be no question that the discrepancy lies with the original data rather than in the 
numbers which have been used to refer the light of Sirius to the Moon. 
The proportion of 9 to 1, which Wollaston has assigned to the light of Sirius and 
a Lyre by a similar method, is, according to the very carefully conducted researches E 
Seidel, more than twice too large. 
With such discordances in the results of the different experiments, it is of impor- 
. tance to vary as much as possible the methods of investigation, in order to render them 
quite independent of each other, so far as the nature of the subject will permit. The 
three principal sources of difficulty encountered are, — the extreme intensity of sun- 
light; the difference in color between the light of the Sun or Moon, and that of the 
ordinary kinds of artificial illumination; lastly, the want of a constant standard of 
brightness, to which either object may be referred. "The best intermediate standard, as 
regards the quality of the light, would undoubtedly be some form of the electric light, 
if its constancy could be maintained for a sufficient interval; all other artificial stand- 
ards have greatly inferior intensity, and a more or less decided red or yellow hue when 
contrasted with solar light, and even with moonlight, which was scarcely to have been ` 
anticipated. | 
* Seidel, Untersuchungen über die Lichtstürke der Planeten Venus, Mars, Ae, p. 35. 
VOL. VIII. 38 
