on the local arrangement of the celestial bodies in space. 313 
lised, which ought to be as nearly as possible equal to that of 
the standard star. At great elevations some difference in the 
altitudes of the stars to be equalised may be admitted ; but, if 
they are far from each other, the circumstance of the equal 
illumination of the heavens, and the equal clearness of the 
air, must still be attended to. 
VI. Of the extent of natural vision. 
The method of equalising star light may be rigorously 
applied to ascertain the extent of natural vision ; for in this 
case it will not be required that the star on which the expe- 
riment is tried, should be of the same size or diameter with 
the standard star; nor is it necessary that the intrinsic bright- 
ness of the light of the two stars should be the same in both. 
It will be sufficient, that the star we choose for an equalisa- 
tion is one of the smallest that are still visible to the natural 
eye. It is also to be understood that, till we can have a well 
ascertained value of the parallax of any one star of the first 
magnitude, the extent of natural vision can only be given in 
a measure of which the distance of the standard star is the 
unit. 
The following equalisations were made in August and De- 
cember 1803, and February 1814,, and are given as a speci- 
men of the method I have pursued. 
Taking Arcturus for the standard of an experiment, 1 
directed the telescope, with one quarter of its light, upon it ; 
while the equalising telescope, with all its light, was succes- 
sively set upon such stars as I supposed might be at double 
the distance of the standard star ; which, as Arcturus is a star 
S s 2 
