59 
Mr. Pond on the parallax of a . Lyrcz. 
* 
treme seasons, and it is not at all improbable that irregular 
refraction, arising from the unequal state of the tempera- 
ture within and without the Observatory, may have had 
a considerable share in occasioning the Dublin discordances, 
combined, perhaps, with the effect of the changes of tem- 
perature upon the instrument itself. It is a circumstance not 
hitherto sufficiently noticed by astronomers, that there are 
many cases where the smallest disturbing cause will produce 
an error quadruple of its own amount ; and consequently, that 
the greatest error to which we are liable from such a cause 
at any one observation will be only one-fourth of the diffe- 
rence that we can detect between the most discordant of them. 
Of such a nature are those disturbances which, like refraction 
for instance, introduce errors, both positive and negative, into 
the determination of either extremity of the arc that measures 
the distance between two stars. < 
By a singular combination of circumstances, not probable 
certainly when considered a priori , but by no means impossi- 
ble, the variation caused by change of temperature may follow 
an annual law so little differing from that of parallax, as to 
bring out the assumed parallax, and to leave the solar nuta- 
tion disengaged. 
Notwithstanding the importance of these investigations to 
the history of astronomy, and to our forming a correct notion 
of the system of the universe, yet our decision ultimately 
turns upon so very small a quantity, that our having reduced 
the enquiry to these narrow limits, rather tends to show the 
perfection of each instrument than the defect of either. 
On former occasions I considered the question of parallax in 
the particular case of a. Lyrae as undecided, and as perfectly open 
to future investigation ; but the observations of the present year 
