the effects of parallax, &c. of cert cun fixed stars. 331 
good, as will appear by Tables 4 and 5. A continuation of 
observations will, I hope, enable me to speak with confidence 
as to the identity or diversity of these numbers. 
The constant for 2 Aquil^ will not be considered of so 
much weight as those of the higher stars, both on account 
of the more uncertain effects of refraction, and because only 
half the effect of aberration is visible in declination ; although 
the influence of these circumstances is somewhat lessened by 
the greater number of observations. 
The investigation of the constant of aberration by direct 
observations of zenith distance has not, that I am aware of, 
been attempted since those of Bradley, by the zenith sector. 
A century has nearly elapsed since his excellent observations 
were made. The results of M. Delambre’s investigations, re- 
lative to the velocity of light, as deduced from the eclipses 
of Jupiter’s satellites, appeared to confirm in so strong a 
manner the mean of Bradley’s results, that astronomers 
seem to have considered the point quite settled ; but if I mis- 
take not, one cause for this was the paucity of instruments 
adequate to so delicate an enquiry. 
In considering the results with a view to the question of 
parallax, whether those that appear to point out parallax 
have not an origin in some cause unconnected with parallax : 
the first remark that offers itself is, that all the results fur- 
nish a positive parallax, if we except those small quantities 
in three of the stars which are quite within the limits of the 
unavoidable errors of observation. Might it not be expected 
that some of the stars would have furnished negative, as 
great as the positive quantities furnished by others ? A con- 
siderable negative parallax would have been decisive. Again, 
MDCCCXXI. U u 
