232 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
I afterwards found to be scarcely distinguishable in brilliancy from the whitest paper. 
This was from two to three times more energetic in its action on the plate than the 
Moon. On repeating the experiment July 2, 1860, with the Sun at an altitude of 
above 65°, and in a clear sky, it was concluded that the same object had about four 
times the intensity of the light of the full Moon. 
The very remarkable photographic power possessed by the rays of Jupiter, which a 
comparison of these results with others already detailed obliges us to admit, naturally 
suggests the enquiry, whether the visual rays from this planet present any similar 
phenomenon. Before considering this subject, however, it will be convenient to explain 
the method followed in the reduction of the observations. 
Let I represent the whole quantity of light emitted, uniformly in all directions, 
from any luminous point; r, the distance from I of a sphere s. 
The light from I being dispersed equally in all directions, every point on the, con- 
cave surface of a sphere circumscribed about J with a radius r, will be uniformly illu- 
minated. If s seen from I subtends an angle 2h, the proportion of the light of I 
which it will intercept will be 
(1) | i — I sin? i A 
If p is the radius of s, we have when p is small 
ubi. fig > 
(2) sin. 5 h=5 75 i=j pl 
But if s is not a sphere, we have 
l , I p 
(3) ier = 
p being the projection of the surface of s exposed to the light upon the concave; or, 
when A is small, its projections as seen from J upon a plane perpendicular at s to the 
line joining J and s. 
The quantity reflected back from s in a given direction will be determined by the 
nature of its surface. A certain proportion, represented by 1 — , will be absorbed, 
leaving oi for he whole quantity reflected. 
The ratio 
(4) whole amount of light reflected from s 
= whole amount of light incident upon s 
is called the albedo of the surface. 
Let ds’ be an element of surface illuminated by light reflected from $, 4 its distance 
from s, d p its projection upon a plane perpendicular to the line. joining s and ds’, and 
