on the Atmosphere of Venus. 311 
1. This diminution of light naturally shews itself between 
the greater eastern and western elongations and the inferior 
conjunction, when Venus appears about halt full, or falcated, 
but still of sufficient breadth. 
2. But this diminution is not equally sensible in all the 
phases of the planet, but sometimes much more so than at 
others. These appearances depend greatly on the clearness 
of our atmosphere, on the telescope giving a very distinct, 
soft, mellow image, and on the eye being properly prepared 
for such observations. 
3. When, in the more falcated phases, we wish to observe 
distinctly and accurately this diminution of light at the farther 
extremities of the cusps, it is necessary to advert that, whilst 
about the middle of the terminating border between d, e, 
where the luminous part has its greatest breadth, this diminu- 
tion, if seen under favourable circumstances, is too perceptible 
to leave the least doubt or suspicion, the points of the cusps, 
especially when the w’eather and other collateral circumstances 
are not favourable, appear, indeed, somewhat fainter, but yet 
almost as bright as the outward limb near b. As the diminu- 
tion of light, whether the cause proceed from its own nature, 
or from that of the atmosphere, must be in the same propor- 
tion at the cusps as at the middle of the terminator, I could 
not but be puzzled at this appearance ; but the multitude of 
observations I have since made on the points of the cusps, 
sometimes when they were tolerably bright, and at others 
when they appeared as faint as the other parts of the said 
border, at length cleared up my doubts, and led me to the 
true cause of this variable appearance, which, to prevent simi- 
lar perplexities in others, I shall here state at length. 
S s 2 
