268 ON THE LIGHT OF THE MOON AND OF THE PLANET JUPITER. 
The telescope was kept in motion by the driving clock during the intervals of obser- 
vation. a 
Jupiter, Screen to focus, 3.0 
« A aJ 
Telescope directed to the Moon and set so-that the focus-aperture included a region 
south of the Mare Fecunditatis, thought to be as bright as the average of the brighter 
parts of the Moon. 
: ft. 
S. of Mare Fecunditatis, Screen to focus, 3.83 
Brighter region, = " 4.33 
Dark — Mare Crisium, Y * 3.33 
Near Mare Crisium, a little north of it, considered to be as bright as any part of 
the Moon. 
ft. 
N. of Mare Crisium, Screen to focus, 4.50 
Jupiter, 4 « 9.67 
113 “ “ 3.67 
The surrounding illumination was the same for both objects in the above experiments. 
Subsequently the same diaphragm was placed in the focus of an eye-piece having a 
power of 206, and the planet and a disc of the Moon of equal angular diameter were 
compared by the eye alternately. It was evident that the color of the two was nearly 
similar, and that, while the intensity of the Moon’s most brightly illuminated region 
exceeded that of the planet, the brighter Seas were about equal to it, and some 
were darker. 
The brighter parts of the Moon selected for the above comparisons were near the 
selenographic meridian passing through the Sun, and were therefore illuminated at the 
maximum angle of incidence. 
1860, March 15th. 8" to 10", m.s.t. Fine clear sky. Cloudless and calm. 
Examined Jupiter at a high altitude near the meridian, with screen glasses of differ- 
ent tints interposed between the eye and the eye-piece of the 23-foot refractor, full 
aperture. Power, 206. Focus aperture as above, 32".47. It was proposed at some 
future time to employ the same screens upon the Sun and Moon. 
After keeping the eye in a dark room for several minutes, the light of the planet 
could be discerned with ease through screen-glasses D and E, both deep red; certainly, 
but not always without some difficulty, through A+ M, A a steel blue, dense, 
and M a thin glass, tint of Madeira wine. B and C, both bluish, admitted glimpses, 
but not with entire certainty. 
1860, March 16th. 7" 30" to 8" 30", Sid. time. Fine clear sky. Eye-piece, focus 
aperture, &c., as on 15th. 
