HALL, ABERRATION CONSTANT. 



43 



tailed; aiul wIhmi the tinve screws which clamp the arms of the micro- 

 sc()|)e are looseiunl, llu' anus can slide sniootliiy around the whole disc, 

 aud may be fastened with the }j;reatest ease and with perfect stability to 

 any point of the disc. IJy rc^ason, however, of the width of the part of the 

 arm which is clamped to the disc, the smallest distance within which any 

 two microscopes can be brought is about ;>0 . 



With the microscopes in Iheir usual position, 1)0° distant from each 

 other, I determined first the error of the line 180°, taking that of the line 

 0° equal to zero; and by bisecting the two arcs betwe(Mi the line 0° and tlie 

 line of 180° corrected, 1 found the eri-ors of the lines 1)0° and 270°. Then 

 one microscope was placed at a distance of exactly 45° from the first; and 

 bv means of these two. and the other three microscopes at the respective 

 distances of 00°, 180° aud 270" from the first, I found the errors of the 

 lines 45°, 135°, 225° and 315°, by bisecling- arcs of 1)0" and 270° corrected 

 according to the former observations. These observations of the lines at 

 a distance of 45° were taken on eight different days, in temperatures rang- 

 ing from 20° to 4fi° Fahi-eiiheit. For each line, two observations were 

 made, one immediately after the other; and the errors given in the follow- 

 ing table are each deduced from the mean of two observations. 



The errors of the lines distant 45° from each other, thus obtained on the 

 difterent davs mentioned, are as follows: — 



The differences in the errors of the same line, found on different days, 

 must have been caused by the different action of the temperature on 

 different parts of the circle. 



The same observations were repeated in the same manner, but in posi- 

 tions of the circle 180° different from those in the former series ; and the 

 following results were obtained, each result being the mean of two obser- 

 vations : — 



