1822.] Finite Exteni of the Atmosphere. 255 
Mercury on the 31st of March of the same year, when he was 
seen at about 65’ from the sun’s centre. 
If I were to describe the little telescope with which my obser- 
vations 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 ata distance 
from the sun ; and, since the principal obstruction to seeing her 
nearer (when the atmosphere is clear), arises from the glare of 
false light upon the object-glass, the success of the observation 
depends entirely on having an effectual screen for the whole 
object-glass, which is obviously far more easy to accomplish in 
the smaller telescope. 
Since the screen which I employed was about six feet distant 
from my object-glass, a similar protection for an aperture of five 
inches would have required to be at the distance of thirty feet, 
io obviate equally the interference of the sun’s light at the same 
period; but this is a provision with which regular observatories 
are not furnished for the common purposes of astronomy. 
As I hope at some future time to avail myself of a larger 
aperture for such observations, without the necessity of mounting 
a more distant screen, it may be desirable that I should suggest 
to others the means by which this may be effected, if they think 
the question of a solar atmosphere worthy of further investi- 
gation. 
If an object-glass of four inches aperture be covered, so as to 
expose only a vertical slit of its surface one inch in width, the 
surface of glass to be so used is about five times as large as the 
circular aperture one inch in diameter, and yet will be as com- 
pletely shaded by a vertical screen at any given distance : and 
an interval of only five feet might allow a star or planet to be 
seen within a degree of the sun’s disc. 
When the sun and planet have the same declination, the ver- 
tical position of the slit is manifestly the most advantageous that 
could be chosen on the meridian; but, for the purpose of seeing 
to the greatest advantage when the line of the centres is inclined 
to the horizon, it would be requisite to have the power of turning 
the slit and screen together at right angles to any line of direc- 
tion of the centres. 
The only fixed star sufficiently near to the ecliptic, and bright 
enough to give any prospect of its being seen near the sun, is 
Regulus, which passes between the 20th and 21st of August, 
but I have not yet had an opportunity of ascertaining within 
what distance from the sun this star can be discerned. 
In the foregoing remarks, I have, perhaps, dwelt more upon 
the consideration of the solar atmosphere, than may seem neces- 
