of ascertaining the Magnitude of small celestial Bodies. 61 
The night is so clear, that I cannot suppose vision at this 
altitude to be less perfect on the stars, than it is on day objects 
at the distance of 800 feet in a direction almost horizontal. 
Oct. 1 1 . By comparing the asteroid alternately and often with 
equal stars, its disk, if it be a real one, cannot exceed 2, or at 
most 3-tenths of a second. This estimation is founded on the 
comparative readiness with which every fine day I have seen 
globules subtending such angles in the same telescope, and 
with the same magnifying power. 
44 The asteroid is in the meridian, and in high perfection. I 
“ perceive a well defined disk that may amount to 2 or 3-tenths 
“ of a second ; but an equal star shows exactly the same ap- 
“ pearance, and has a disk as well defined and as large as that 
“ of the asteroid.” 
RESULT AND APPLICATION OF THE EXPERIMENTS AND 
OBSERVATIONS. 
We may now proceed to draw a few very useful conclusions 
from the experiments that have been given, and apply them 
to the observations of the star discovered by Mr. Harding ; 
and also to the similar stars of Mr. Piazzi and Dr. Olbers. 
These kind of corollaries may be expressed as follows. 
( 1 . ) A 10-feet reflector will show the spurious or real disks, 
of celestial and terrestrial objects, when their diameter is ~ of 
a second of a degree ; and when every circumstance is fa- 
vourable, such a diameter may be perceived so distinctly, that 
it can be divided by estimation into two or three parts. 
( 2. ) A disk of ^ of a second in diameter, whether spurious 
or real, in order to be seen as a round, well defined body* 
