96 
Dr. Wollaston on the 
ever, 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 atmo- 
sphere 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 dis- 
tant from my object-glass, a similar. protection for an aper- 
ture of five inches would have required to be at the distance 
of thirty feet, to 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 pur- 
poses 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 mea.ns by which this may be 
effected, if they think the question of a solar atmosphere wor- 
thy of farther investigation. 
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 completely shaded by a vertical screen at any given dis- 
tance : 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 
vertical position of the slit is manifestly the most advantage- 
