1875.] The Atmospheres of the Planets. 465 
from the limb, it would be utterly insensible for any possible 
value of 6, the density of the atmosphere at the surface 
of Mars, which cannot be supposed to be much more 
than 5, but is probably slightly lessthan unity. A distance of 
one-third of a second of arc from the true limb of Mars is to 
all intents a quantity perfectly indistinguishable, for the 
irradiation at the border of the planet must usually exceed 
this, so that no effects of refraction from the atmosphere of 
Mars during an occultation of a star can be expected to be 
telescopically visible. It will also be apparent that extreme 
limits of what can in any sense whatever be held to be an 
atmosphere to Mars must lie within two-thirds of a second 
of arc from the true limb of Mars. 
In certain features the atmosphere of Venus seems to 
present a strong contrast to that of Mars; thus en the 
latter extensive dense masses of cloud are very exceptional, 
but on Venus they appear to be the rule, so that it is seldom 
that the true surface can be distinétly seen, whilst the earth 
appears to possess intermediate features. The evidence 
establishing the actual existence of an atmosphere to Venus, 
though of different character to that referring to Mars, is 
yet even stronger, depending principally upon the effects of 
the refraction exerted by it. The first observation pointing 
to the considerable horizontal refraction of Venus was an 
observation by Andreas Mayer, who observed the planet when 
near inferior conjunction ; the horns of the crescent being 
prolonged until meeting, the dark planet was surrounded by 
a ring of light, throwing the dark body into relief. Since 
then the planet, when near conjunction, has repeatedly been 
seen as a dark body surrounded by a bright ring; whilst 
the prolongation of the horns of the crescent planet has, 
under favourable conditions, been noticed by a number of 
observers since Herschel and Schroéter. In 1849 Madler, 
from a series of careful measures of the amount of this 
prolongation, determined that the horizontal refraction 
necessary to produce this was 43’ 42”. Lyman in 1866, 
from similar measures, found it to amount to 45’ 18’, and 
from a further series of observations in 1874 obtained 44’ 30’, 
on both these occasions seeing the dark body of Venus sur- 
rounded by a beautiful silvery ring of light. From these 
three series of observations the horizontal refraction of the 
atmosphere of Venus can be considered as determined to be 
44’ 30” with tolerable certainty, and the density of the 
atmosphere can therefore be determined with a considerable 
approach to accuracy ; for the spectroscopic observations of 
Huggins, Janssen, and Vogel show that this atmosphere is 
