308 Dr. Brinkley’s results of the application of 
west, the results may be considered the same as if they had 
been determined by two mural circles. 
II. The inclinations of the line of collimation of the colli- 
mator, as determined on different days, are given. The 
permanency of this inclination from day to day is by no 
means necessary ; but it taking place, appears to give addi- 
tional value to the collimator ; and the exactness with which 
it can be ascertained, puts beyond all question that the colli- 
mator is applicable to the most powerful instruments. 
III. The index corrections of the circle, as determined on 
different days by the help of the collimator. The uniformity 
of these results is affected by the exactness of the adjustment 
of the vertical axis of the circle by the plumb-line ; and pos- 
sibly the line of collimation of the telescope of the circle, at 
least the index correction, may suffer occasional changes by 
a very small derangement in the parts of the instrument, so 
as to vibrate as it were about a mean state. 
IV. The index corrections, as determined by reversing 
from several stars. These, as well as the index corrections 
determined by the collimator, are affected with the errors of 
division. 
It may serve for illustrating the importance of the floating 
collimator, to consider that it furnishes the observer with a 
fixed star, which he can place in his horizon (in fact at any 
given altitude), unaffected by refraction, visible at any instant, 
and on which he can make his observations in a closed room. 
That this fictitious star can be observed with as great exact- 
ness as a fixed star of the first magnitude under the most 
favourable circumstances of daylight. I cannot but think, 
had such a discovery been announced, and the means of 
