480 
1 Stereoscopic with approximate confidence interval. 
J Monocular with — 
j» Binocular -■— — m— 
1. Stereoscopic observation 
With the aid of the Dove-prisms the y-parallax has been made appar 
ently parallel to the eyebase, and observed stereoscopieally. 
2. Binocular observation. 
The Dove-prisms are in an ordinary position and the y-parallax is 
perpendicular to the eyebase. The images are observed simultaneously 
and the y-parallax is eliminated when the mark is made as distinct as 
possible.