110 
SIR W. DE W. ABNEY AND PROF. AY. AYATSON ON 
threshold observations using (l) both eyes. (2) the right eye only, and (3) the 
left eye onty. Sets of observations were made both at the fovea and at 5 degrees from 
the fovea. They both found that it was much more difficult to obtain satisfactory 
SSN 
Fig. 13. 
settings when using only one eye. The difficulty appears to be due to not being able 
to keep the accommodation of a single eye so adjusted that the fixation light 
is always in focus, unless so bright a fixation light is' used as to interfere with 
c: 
< 
310 4l0 5l0 610 
SSN 
Fig. 14. 
the extinction settings. The actual annulus readings obtained are plotted in 
tigs. 13, 14, and 15. It will be seen that for W. at 5 degrees from the fovea the 
