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SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
10 degrees. For B., while the threshold at the fovea is slightly lower than for the rest 
of the retina, the values obtained at the other distances are very nearly the same, 
indicating that over this region of his retina the sensitiveness remains practically 
constant for light of very low intensities. This peculiarity that the sensitiveness 
of B. s retina for feeble lights is a maximum at the fovea has some interesting con¬ 
sequences. Thus he finds no difficulty in obtaining central fixation and he can obtain 
luminosity measurements even with very feeble intensities which are quite consistent. 
With W., on the contrary, central fixation with feeble lights is difficult, and when he 
attempts to make luminosity measurements at low intensities his results are very 
irregular, owing to the tendency to use the parafoveal regions of the retina, which are 
more sensitive than the fovea. Adopting as beforesaid for convenience yon Kries’s 
