160 Dr Wollaston m the Finite Extent of the Atmosphere. 
light upon the object-glass, the success of the observation depends 
entirely on having an effectual screen 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 aperture 
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 obser- 
vatories are not furnished for the common purposes of astrono- 
my. 
‘‘ As I hope at some future time to avail myself of a larger 
aperture for such observations, without the necessity of mount- 
ing 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 farther 
investigation. If an object-glass of four inches aperture be co- 
vered 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 
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 
vertical 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 direction 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 
Begulus, 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.” 
