316 Dr. Brinkley’s results of the application of 
By a comparison of the inclinations of the line of collima- 
tion of the collimator, as determined in October and Novem» 
her by both the fine wire and thick wire, it will be seen that, 
as far as errors of observation are concerned, an error of a 
second rarely takes place. It may therefore serve to illus- 
trate the powerful optical principle, on which the application 
of the collimator is founded, to remark that, as the collimator 
furnished by the Board of Longitude is only about 13 inches 
long, a space of less than ^oioo °f an ascertained ; 
for to such a degree of exactness the position of the cross 
wires of the collimator can be ascertained. Thus perhaps a 
most severe test of a micrometer screw might be furnished. 
J. BRINKLEY. 
