C sO 
II, Experiments for ascertaining how far Telescopes will enable us 
to determine very small Angles , and to distinguish the real from 
the spurious Diameters of celestial and terrestrial Objects : with 
an Application of the Result of these Experiments to a Series of 
Observations on the Nature and Magnitude of Mr. Harding’s 
lately discovered Star. By William Herschel, LL. D. F. R. S . 
Read December 6 , 1804. 
The discovery of Mr. Harding having added a moving 
celestial body to the list of those that were known before, I 
was desirous of ascertaining its magnitude ; and as in the ob- 
servations which it was necessary to make I intended chiefly 
to use a ten-feet reflector, it appeared to me a desideratum 
highly worthy of investigation to determine how small a dia- 
meter of an object might be seen by this instrument. We know 
that a very thin line may be perceived, and that objects may 
be seen when they subtend a very small angle ; but the case I 
wanted to determine relates to a visible disk, a round, well 
defined appearance, which we may without hesitation affirm to 
be circular, if not spherical. 
In April of the year 1774, I determined a similar question 
relating to the natural eye : and found that a square area could 
not be distinguished from an equal circular one till the diameter 
of the latter came to subtend an angle of 2! 17". I did not 
think it right to apply the same conclusions to a telescopic view 
