56 
Dr. Brinkley on the 
refractions, and so made the Catalogues differ. He attributes 
the differences to flexure. Now he admits that the flexure 
would be the same at equal distances on each side of the 
zenith ; but it does not appear to have occurred to him that 
my refractions were determined by observations of circum- 
polar stars to the north of the zenith by the same instrument, 
and that therefore they must be exactly in error by the quan- 
tity of flexure ; and so when applied to stars south of the 
zenith, must exactly compensate for the effects of flexure.* 
Mr. Pond did not perceive that what he took away with one 
hand, he ought to have restored with the other, and so left 
my Catalogues as he found them. 
It is difficult to say how far the difference of our constants 
of refraction may be occasioned by a discordance in the 
meteorological instruments. This should be enquired into. 
It is still more difficult to imagine a difference in the mean 
refractions at the two places. 
* The manner in which the telescope and circle are attached in the Dublin in- 
strument, appears to preclude all probability of flexure in the telescope. Indeed 
it does not appear a matter of much difficulty where the telescope and circle are 
combined together, as in the Dublin and Greenwich instruments, to guard against 
a flexure in the telescope. If talents such as those of Mr. Ramsden and Mr. 
Troughton have been unable to provide against the flexure of the telescope, it 
appears to me quite useless to expect exactness in the other parts of the instru- 
ments. Therefore, it might be considered as almost a waste of time to endeavour 
to overcome the difficulties I should have to encounter here by observing by reflec- 
tion. The difficulties in general would be greater than at Greenwich ; and, above 
all, among the few clear days that occur, very few could be found sufficiently calm 
to observe by reflection. 
Mr. Pond had a motive for pursuing this mode of observation which does not 
exist here ; he had no other method of determining his zenith point. I do not con- 
sider the observations by reflection necessary for my own satisfaction, but if they 
be for that of others, I should not object to undertake them. 
