938 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
intensity of the illumination of the phase from the edge presented towards the Sun up 
to the boundary between the bright and dark hemispheres. It is interesting to note, 
in the photographs of the Moon and Venus, when half enlightened, how much more 
sensitive to this graduation of intensity is the photographic plate than the eye. 
Mr. De la Rue remarks that *the lunar surface, very near the dark limb, is copied 
photographically with great difficulty, and it sometimes requires an exposure five 
or six times as long, to bring out completely those portions illuminated by a very 
oblique ray, as others apparently not brighter. * The comparative feebleness of the 
edge near the dark part of the disc I have found to be most striking while the picture 
is developing. 
He considers this to be an indication that there is an inequality in the chemical 
action of different parts of the Moon equally bright to the eye; and indeed it is difficult 
to convince one's self, from the impression produced on the retina, that the neighbor- 
hood of the unenlightened edge is relatively as deficient in optical as it is in chemical 
intensity. It is plain, however, that this inequality actually exists. If we view the 
Moon at quadratures when half enlightened, the quantity of sunlight incident on an 
element of its surface d's, placed so that its projection 
a 2. dp falls at P, on the semi-diameter C E, Fig. 2, will 
Sen e vary-as the cosine of the angle between two lines, 
E DX. one drawn from the centre to the surface at E, where 
fe X the Sun is in the zenith, and the other to the element 
ds. There is then a considerable region near the dark 
limb where the average visual intensity is not greater 
/ than one fifth or one sixth of that of the bright limb, 
/ justas is found to be the case photographically. Ob- 
a E servations, which will be noticed in a subsequent part 
se Rc en of this Memoir, prove that this diversity actually oc- 
curs in the distribution of light over the half-Moon, 
in accordance with the above theory, as well as with the photographic results. 
The photographs of Venus exhibit the gradations of illumination over its sur- 
face E bene The eye, indeed, with some attention, recognizes, in the tele- 
scopic view, a brightening up of its light on the limb next the Sun, but the contrast 
comes out much more decisively in the photographs. 
It has been observed by De la Rue, that the high ground of the Moon's 
southern 
* Report of the British Association, 1859, p. 145. 
