598 Prof. Brace on the Change produced by the 



settings with the light going westward into the observing 

 telescope, and then, on reversing, five settings with the light 

 entering the eye from the west, or vice versa. I then rested 

 my eye preparatory to a second set, and so on. The settings 

 for a match were necessarily made by myself, my assistant 

 reading the drum of the micrometer-screw and recording the 

 same, and then reversing the trough steadily so as not to 

 displace my eye, which was kept focussed upon a definite 

 point of the vanishing line. Some 250 settings were made 

 at the noon hour, and 100 at 5.30 p.m. sun-time ; part being 

 taken on reversal from West to East, and part from East to 

 West. 4'2 turns of the micrometer-screw corresponded to 

 5° of arc. The drum of the micrometer-screw was divided 

 into 50 parts. 



I give below a single set of readings taken at random, and 

 also the differences W— E in the means of each set of five 

 readings. 



Micrometer Settings 

 in divisions of the drum. 



w. 



E. 



\ 





13-0 



13-8 







13-4 



13-6 







13-3 



13-2 







13-4 



13-4 







13-2 



13-4 







13-26 



13-48 







Differences 



in 







Micrometer Settings. 





Aug. 3, 1905. 

 { 11 h. 53 m. to 12 h. 18 w. 



W-E. 







Reversal West to East. 



- -20 









- -22 









+ -16 









+ •27 









+ •64 









+ -54 









•00 









- -22 









+ -56 









+ -46 





/ 





— •06 







) 



- -38 







Aug. 3, 1905. 



•00 







V 5h. 13 m.— 5 h. 23 m. 



- -34 







West to East. 



+ •12 







1 



