Dr Wollaston on the Finite Extent of the Atmosphere. 159 
when his distance was only 65'; and in both these cases,, the 
observed and the calculated positions agree exactly. 
These arguments receive, if they required it, additional 
strength from the phenomena of the eclipses of Jupiter’s satel- 
lites. These ‘ bodies advance regularly, and without any retar- 
dation from refraction, to the very disc of the planet ; so that 
Jupiter cannot possess that extent of atmosphere which he is ca- 
pable of attracting to himself from an infinitely divisible medium 
filling space. For, taking Jupiter’s mass at 309 times that of 
the Earth, and his diameter at 11 times that of the Earth, then 
. . 17.6 
V309 = 17.6 times the Earth’s radius, r= =1.6 times his 
own radius, which will be the distance from his centre at which 
an atmosphere equal to our own should produce a refraction of 
one degree. To the fourth satellite this distance would subtend an 
angle of about 3° 37' ; so that an increase of density to 3 J times 
our common atmosphere, would be more than sufficient to ren- 
der the fourth satellite visible to us when behind the centre of the 
planet^ and consequently^ to appear on both (or all) sides at the 
same time. The space of about 6 inches in depth within which 
this increase of density would take place, would not subtend so 
much as of a second. 
Hence, Dr Wollaston concludes, that all the phenomena ac-- 
cord entirely with the supposition that the EartlCs atmosphere is 
of finite extent^ limited by the weight of ultimate atoms ^ of defi- 
nite magnitude^ no longer divisible by repulsion of their parts. 
Having thus stated Dr Wollaston’s reasoning as perspicuous- 
ly as we can, we shall lay before the reader, in his own words, 
a description of the apparatus which he employed. 
“ If I were to describe the little telescope with which my ob- 
servations were made, without taking due care to explain the 
precautions adopted, and the grounds of their efficacy, it might 
perhaps be scarcely credible, that, with an object-glass less than 
one inch in aperture, having a focal length of only seven inches, 
I could discern an object not to be seen by telescopes of four 
and five inches aperture. We know, however, that this small 
aperture is abundantly sufficient for viewing Venus at a distance 
from the Sun ; and since the principal obstruction to seeing her 
nearer, when the atmosphere is clear, arises from the glare of false 
