ON THE CHANGES OF THE MOON’S SURFACE. 515 
one but himself was able to give. These volcanoes we now know were the 
strong shining Ring-mountains, Aristarchus, Copernicus, and Kepler, which 
he perceived on the dark part of the moon, and which every one who pos- 
sesses a telescope of sufficient power may also see at the approach of every 
first quarter of the moon. They always appear the same, with the excep- 
tion of the small variations caused by libration; and I have never missed 
them even in total eclipses, finding them even then bright and distinct 
enough to convince me by measurement that they were identical with those 
well-known Ring-mountains. This also would not have escaped the notice 
of Herschel, if the important and varied undertakings, of which he alone was 
capable, had left him time to attend more especially to the moon. Schréter, 
of Lilienthal, put forward to occupy the field left free by Herschel. With 
telescopes of dimensions little less than Herschel’s, he observed zealously and 
attentively the surface of the moon, and his “ Selenotopographical fragments” 
for a long time excited general attention, though they are now almost for- 
gotten ; in fact, it cannot be denied that he mainly promoted the science by 
gaining Harding and Bessel to astronomical pursuits, and providing them 
with means to dedicate themselves entirely to its service; for his instru- 
ments, which for a long time had crowded up the Gottingen Observatory, 
have after a close examination by Gauss been pronounced almost useless, and 
the telescopic mirrors which were presented to the Cabinet des Physiques are 
all that now remain. The observations were never collected, neither were 
his data offered to others for the purpose of being reduced to mean librations : 
and besides, he paid too little attention to the advice of his renowned friend 
Olbers ; he would discover variations on the moon’s surface, without found- 
ing on them a moon map, for he expressly declared that he regarded them 
as useless. My endeavours to obtain from his sketches a connected picture 
of the moon, or at least of a part of it, were in vain; and Bessel has shown 
the inaccuracy of his measuring-apparatus by giving in one design of Schroter 
42" and in another 89" for one and the same distance. Kunowsky has 
proved that all these variations which he pointed out were mere illusions. If 
we desire to arrive at unmistakeable conclusions respecting physical changes 
on the moon’s surface, it is imperatively necessary to bear in mind the 
optical variations, which may consist 
(1) In displacement by libration. 
(2) In the different illumination by the sun. 
(3) In the changeable transparency of our atmosphere. 
(1) Mountains situated nearer the centre than the edge of the moon’s disk 
in appearance are but little affected by libration; the nearer the edge the 
more they are affected. In such a position a crater might be easily concealed 
by one of its sides, so that one might suppose it only a mountain; whilst by 
a libration, which removes it further from the edge, this concealment does not 
take place, and we get a sight of the crater, ¢. g. that of Schréter’s newly de- 
scribed crater in the Ring-mountain. evel is probably so to be explained, 
(2) Mountain-walls throw shadows as long only as the angle of the incli- 
nation is greater than the height of the sun above their horizon ; and more- 
over in places situated far from the centre of the moon, the shadow of a 
declivity is to our view concealed by the declivity itself; but the smaller 
mountains are only to be recognized by their shadows, and a whitish shadow- 
less spot may always be circular and sharply defined without our being able 
to distinguish whether here a crater, a mountain, or neither is exhibited. 
Most of the moon-mountains demand of us repeated observations if we would 
gain a right judgment respecting them. 
