on the Atmosphere of Venus, 313 
ee seen with certainty at such times when the air is very clear 
and calm, and a good telescope with a large aperture, and high 
magnifying power, is used in the observation. And it follows 
that, except under those circumstances, the parts of the cusps 
af , ch, must, on account of their great sharpness, be the ap- 
parent and sufficiently luminous extremities ; since, according 
to well known principles, the exterior points a, and c will 
have as much light as the point k, near the middle of the 
crescent. 
But in a clear and calm atmosphere, and with a high mag- 
nifying power, it is truly pleasing to see, after the eye is ac- 
customed to it, how the whole of the terminating border, even 
to the farther extremities of the cusps, vanishes gradually, and 
becomes at last so faint, that in the day time, and where there 
are any inequalities, it insensibly loses itself in the colour of 
the sky. Such striking diminutions of light have I seen re- 
peatedly with my four-feet reflector, with a power magnifying 
280 times, arid my seven-feet reflector, with a 370 magnifying 
power ; and particularly on the 20th of November, 1791, when 
with a power of 161, I saw the light of the terminator dwindle 
away, and appearing, for a breadth of about 1 or i~ seconds, 
almost as grey as the ash-coloured spots on the moon. 
Those who are at all acquainted with the theory ofllight, 
need hardly be reminded that on an illuminated spherical sur- 
face of a planet, the light will ever appear fainter towards its 
border, in proportion as the angle between the incident ray 
from the sun and the said surface becomes smaller.* 
* It may equally be superfluous to illustrate how, since the sun appears on the 
planet Venus under a diameter of about 44', its luminous border must have a no in- 
considerable penumbra. It appears, however, from calculation, that this whole pe- 
