57 
Mr. Pond on the parallax of a Lyrx. 
introduced itself into this series of observations ; for I obtain 
the same result from the readings with two microscopes as 
from those made with six. 
In the case of two microscopes, the angular distance is mea- 
sured upon two arcs only. Now it cannot be for a moment 
contended that an error from temperature, so great as not to 
be corrected by six microscopes, will not be much exagge- 
rated by employing only two. The errors then, if any, must 
arise from the effects of temperature on refraction, and not 
from the changes it occasions in the instrument. But from 
the season which I have chosen for this investigation, and from 
the care that has been taken to equalize the temperature, the 
errors arising from the latter cause must be almost insensible. 
My observations, thus conducted, indicate in the most decided 
manner, that the parallax of a Lyrae cannot exceed a very 
small fraction of a second. The advantages and disadvantages 
of the Dublin and Greenwich methods are in this process 
much more nearly balanced than in the former. The Dublin 
instrument has the great advantage of determining the zenith 
distance in the course of a few minutes, whereas at Green- 
wich twenty-four hours at least, and frequently several days 
elapse, before a complete observation of the double altitude 
can be obtained by the method of reflection. This disadvan- 
tage attending the Greenwich method could only be remedied 
by employing two mural circles for observing a star on the 
same night, both by direct vision and by reflection. 
I have now to consider that argument on which the great- 
est reliance in favour of parallax has been placed, namely, 
that founded on the actual determination of the solar equation 
from the observations made with the Dublin instrument. 
This argument may, I think, be thus stated. By a series 
I 
MDCCCXXIII. 
