Mr. Schroeter’s Observations 
314 
But what here claims our particular notice, is the singular 
circumstance that, except in the planet Mercury, concerning 
which I have not yet a sufficient number of observations, a 
similar diminution of light is not observed in so sensible a de- 
gree on any of the other planets of our solar system, our earth 
only excepted. 
Mars, Jupiter, and Saturn, cannot, indeed, on account of 
their great distance, exhibit on our globe the variable phases 
of a half, or smaller portion, of an illuminated hemisphere, 
whence no fair arguments can be derived from those instances : 
but the comparative appearances of the moon, in this respect, 
will be thought the more singular if carefully attended to, the 
marginal diminution of light on this satellite, which, however, 
like Venus, is a sphere illuminated by the sun, not being 
nearly so perceptible and evident, as that above described. Of 
this we may fully persuade ourselves, by comparing the ap- 
pearances of the terminating borders of the moon in its fal- 
cated phases or quadratures, with the same borders on Venus 
at the same periodical aspects. Should this striking difference 
not be reconcileable upon our established optical principles, 
nothing will remain but the analogy, that, as the density of 
our atmosphere checks the sun-beams the more, the longer 
they proceed therein in a direction which, after the rise or be- 
fore the setting of the sun, carries them over a certain track of 
land ; and as such a track, on which the sun at its rising and 
numbra, which, according to the greatest apparent diameter of Venus, extends from 
59 to 60", measures in the direction perpendicular to the line of the cusps only 0,36", or 
i of a second of a degree ; and that where this penumbra is observable in its exterior 
darker parts, its breadth is not above of a second, whence it cannot be visible 
throughout. 
