1875.] The Atmospheres of the Planets. 469 
miles above the surface, far above any surface vapours 
that alone powerfully diminish the solar light and pass 
through a column of atmosphere at its densest point, not 
one-half the density of the air near the earth’s surface. 
The solar rays would traverse, therefore, the atmosphere of 
Venus with scarcely any diminution in intensity, and 
appear thus brighter than the dusky limb of the sun, and 
from the contrast with the dull orange or reddish tinge of 
the solar border appear silvery in hue. 
The rapid disappearance of the very delicate line of light 
after the planet had left the solar disc is a phenomenon not 
very satisfactorily explainable, and must be regarded as pre- 
senting a difficulty. The brilliancy of the light refracted by 
the atmosphere of course very rapidly diminishes with the 
increase of distance from the border of the sun, both from a 
less amount of light being refracted and from its traversing 
a longer and far denser column of atmosphere; but this by 
itself is scarcely sufficient to account for its complete dis- 
appearance. ‘The presence of clouds or mist in the lower 
lying strata of the planet’s atmosphere is of course ade- 
quate to entirely account for the observed phenomena, and 
may well have been its origin; but still it can scarcely be 
regarded satisfactory to have to assume the existence of a 
convenient band of clouds where they were wanted, and 
nowhere else. Probably the disappearance arose partly 
from the weakening of the solar light, but mainly from the 
disappearance of the dark body of the planet, which, when 
visible, would at once direct the eye to the position of the 
delicate ring, and by contrast throw it strongly into relief, 
whilst, when this had toa great extent disappeared, the deli- 
cate ring of light would be a very difficult obje@t to dete@ 
through a dark glass. That the ring of light had not dis- 
appeared is unquestionable ; for it was seen perfect by Lyman 
with great distinctness (without a dark glass) five hours 
before ingress, and still in part a considerable time after the 
transit. 
A point of some interest in connection with this silvery ring 
of light was its forming close to the pole of Venus a bright 
spot, according to the Australian observers, which it has 
been suggested may arise partly from a greater refraction, 
owing to the greater density of the colder atmosphere near 
the poles, combined with reflection from the snow or ice zone 
of the planet. Ifthe atmosphere was fairly free from cloud 
towards the poles, something similar to this might be 
expected ; for the Italian observers under Tacchini have 
ascertained the presence of moisture in the atmosphere of 
