Mr. W. Crookes on the Photography of the Moon. 233 
Appendix. 
Besides the pictures taken in America—which are almost valueless 
as moon maps, as the sides are reversed in the copying from the 
daguerreotype plate upon which they were originally taken,—the 
moon has been photographed by Professor Phillips, Father Secchi, 
MM. Bertsch and Arnauld, several Liverpool photographers, and 
Mr. Hartnup and myself. It is interesting and instructive to 
compare among themselves the means employed and the time occu- 
pied in taking the impression on these several occasions. 
Professor Phillips’s telescope has a sidereal focus of 11 feet, and 
an aperture of 6; inches; consequently the brilliancy of the moon’s 
image in its focus is augmented 26 times over what she appears to 
the naked eye. The average time occupied for the collodion plate 
to receive the impression was about 3 minutes. 
Father Secchi’s telescope having a sidereal focus of 18 times its 
aperture, the moon’s image was intensified 37°8 times, and the time 
required for the impression was an average of 6 minutes. 
M. Porro’s glass of 49 feet sidereal focus and 20 inches aperture, 
gave a moon image 12:3 times brighter than she appeared to the 
naked eye, and the average time of taking the picture was 17 
seconds. 
Mr. Hartnup’s telescope being 12} feet focus and 8 inches aperture, 
augments the intensity of the moon’s image at its focus 35:1 times. 
The time which was required for the photograph of our satellite to be 
taken, on the occasion of the meeting of the British Association 
at Liverpool in 1854, was about 2 minutes; and under the same 
circumstances we ourselves succeeded in obtaining perfect and intense 
negatives in 4 seconds. These, however, were taken under very 
unfavourable circumstances, the temperature being below the freezing- 
point, and the moon at a considerable distance from the meridian, 
which necessarily caused both a diminution of the light and also a 
diminished sensitiveness of the collodion film. 
The rapidity with which the above pictures were taken may be 
better understood by comparing them with those of terrestrial objects 
under similar circumstances. According to Herschel*— 
“The actual illumination of the lunar surface is not much superior 
to that of weathered sandstone rock in full sunshine. I have fre- 
uently compared the moon setting behind the grey perpendicular 
agade of the Table Mountain, illuminated by the sun just risen in 
the opposite quarter of the horizon, when it has been scarcely distin- 
guishable in brightness from the rock in contact with it. The sun 
and moon being nearly at equal altitudes, and the atmosphere per- 
fectly free from cloud or vapour, its effect is alike on both lumi- 
naries.”’ 
Thus by comparing the Liverpool object-glass as to power with 
our ordinary camera lens, its focal length being nearly 19 times the 
aperture, and the moon’s image being copied by its means in 4 
seconds, we find that it is equivalent to copying sandstone illuminated 
* Herschel’s Outlines of Astronomy, page 249. 
