408 
objects of familiar character is judged in 
part by their apparent size—the principle 
of lunar perspective. Distant objects, be- 
ing seen through more air, have a different 
color from near objects—the principle of 
aérial perspective. For objects in the fore- 
ground we have two retinal pictures which 
are sensibly different, and from these the 
eyes estimate distance—the principle of 
optical parallax. The judgments arising 
from optical parallax are automatically 
combined into judgments of the rotundity 
or solidity of objects. If the sun shines, 
many objects show one side illuminated and 
the other side in shadow, and a shadow is 
also cast on the ground or on adjacent ob- 
jects. From long association with judg- 
ments arising from optical parallax we have 
come to infer rotundity from the distribution 
of illumination and non-illumination—the 
principle of shadows. Finally, sunlight is 
scattered and reflected from particles in 
the air and from objects in the landscape 
so as to come with modified intensity from 
many directions, affording partial illumi- 
nation to surfaces not directly exposed to 
the solar rays. This partial illumination 
we have learned to interpret in terms of 
rotundity—the principle of shades. 
When one looks at the surface of the 
moon through a telescope of high power 
he sees a landscape from an unfamiliar 
direction and under unfamiliar conditions. 
Asall the objects are strange, he is not aided 
by linear perspective. As there is no fore- 
ground, he is without the assistance of 
optical parallax. As the moon has no 
atmosphere his view is devoid of aérial 
perspective and of shades. It is true that 
the moon receives reflected light from the 
earth and there must also be reflection from 
lunar cliffs, but these reflected lights are so 
faint as not to help the seeing of surface 
details ; so far as the eye can determine, 
the lunar shadow is absolutely black. The 
observer, being deprived of all other data, 
SCIENCE. 
(N.S. Vou. XIII. No. 324. 
has to depend wholly on light and shadow, 
complete illumination and the entire lack of 
illumination, for his determination of the 
configuration of the surface. The sense of 
sight, having been educated by terrestrial 
landscapes, is unprepared for the peculiar 
conditions of the lunar landscape and gives 
false judgments. Close to the terminator, 
or sunrise line, where light and shadow 
divide the field, the eye overestimates the 
relief and sees the topography as grossly 
exaggerated as some of the published 
sketches of lunar mountains. At a dis- 
tance of 40° or more from the termina- 
tor the landscape is practically without 
shadows, but is diversified by spots of color 
representing the distribution of the various 
substances composing the moon’s face. 
These colors, being chiefly light and dark 
grays, are interpreted by the eye as shades 
and give an impression of relief no less 
false than that obtained at the terminator. 
Along an intermediate zone the general 
effect as to altitude is substantially true, 
but it can hardly be doubted that many de- 
tails of form are misconceived. 
Professor W. Prinz, of Brussels, has hit 
upon an ingenious method of avoiding these 
difficulties and realizing the actual relief. 
For many years the rotundity of the moon 
as a whole has been exhibited by means of 
the stereoscope. The possibility of this 
depends on libration, which permits us to 
view the moon from different directions - 
ranging through an are ofabout 16°. Two 
photographs taken in different months 
and at times properly chosen, and after- 
ward viewed through a properly con- 
structed stereoscope; give the same fulness 
of relief which we obtain in observing an 
object at a distance of 9 inches. Professor 
Prinz has applied the same method to the 
examination of small portions of the lunar 
surface greatly magnified, and is thus en 
abled to see the craters and other details in 
their natural proportions. To get the best 
