214 C. Abbe on the Repsoid Portable Circle. 
By comparing these dimensions with those of the four times 
larger Ertel vertical circle of the Central Observatory, it is evi- 
dent that the portable instrument, for which the actual probable 
accidental error of a meridian zenith distance resulting from two 
pointings, one circle east and one circle west, is 0'5, has de- 
rived a decided advantage from its small size and proportion- 
ately higher magnifyin wer 
Recognizing the principle that in an astronomical measure- 
of reading these divisions, combine to affect the mean of four 
measurements with the microscopes, +017 
vided circle, +0''46, affects the mean of four, +028 
The probable accidental error of a reading of the two 
ends of the level bulb, +012 
The combination of these gives 
(0-17)? (0-23)2--(0"12)?= 031. 
The probable accidental error of pointing on a star ma, 
afterward investigated,—but if we assume that it equals an 4p- 
parent visual angle of 1’, this will correspond to an are of 
+0'"67. Whence 
/(0"31)2--(0"67)2 = -0"74. 
A zenith distance depending on one pointing, circle right, and 
