468 The Atmcspheres of the Planets. {O¢tober, 
of a beautiful silvery ring of light around Venus during 
ingress, enabling the entire disc of the planet to be seen long 
before reaching the interior contact. This very delicate ring 
of light appears to have been considerably brighter than the 
sun’s limb, and forming a fine brilliant very delicate line of 
light round the portion of the dark planet off the solar disc. 
Though this appearance seems to have been quite unexpected 
to the majority of the observers, it is a feature that the 
results of Madler and Lyman showed might be expected, 
and appears indeed to have been recorded by several of the 
observers during the transits of Venus of the last century. 
There can be no question but that it arises from the hori- 
zontal refraction of the atmosphere of Venus, though not 
at first sight quite in accordance with the results already 
obtained with reference to the dimensions of the atmosphere 
of that planet ; for although it has been shown that the 
thickness of the sensible atmosphere of Venus was only 
one-fifth of a second of arc, the breadth of this ring of 
light was estimated to be less than one-second of arc, or 
probably about two-thirds of a second of arc. It is mani- 
fest, however, that this so far from being inconsistent with 
the small breadth of the atmospheric zone of Venus is 
exactly what was to be anticipated for the effects of the spu- 
rious disc arising from diffraction, which would increase 
the apparent breadth of the line of light from a thickness 
of only one-fifth of a second to nearly one second. The 
superior brilliancy of the line of light around Venus to the 
limb of the sun is also easily understood, for it is well known 
that the solar limb is far inferior in brightness to the central 
portion ; but the light refracted in any one direction by Venus 
is composed of rays from every portion of the solar surface, 
and consequently would be far nearer the brightness of the 
central portions of the solar disc than that of limb, and 
even allowing for a considerable amount of absorption by 
the passage through the atmosphere of Venus would be 
still superior in brightness to the limb of the sun. More- 
over, as the horizontal refraction of Venus is so far greater 
than the semi-diameter of the sun, it is evident that the 
principal rays which pass through the extreme low-lying 
strata of Venus’s atmosphere would fail to reach the earth 
when the planet was crossing the solar limb, from being 
refra¢ted too much. It is easily shown that the horizontal 
refraction of Venus’s atmosphere being nearly 45’, the prin- 
cipal solar rays refracted to the earth by Venus’s atmosphere, 
when the planet was crossing the solar edge, would traverse 
the atmosphere of the planet at a height of nearly six 
