122 
SIR W. DE W. ABNEY AND PROF. W. WATSON ON 
5. Visibility of Radiation. 
Having determined the energy distribution for the two sources of light we have 
employed, it seemed of interest to obtain the visibility that is the quotient of the 
luminosity by the energy throughout the spectrum. We have done this in the case 
of the observer W. (who has no rods at his fovea), taking care to use, when making 
the luminosity measurements, a photometer field of such a size that the image formed 
on the retina was confined to the fovea. The luminosity values obtained are given 
in Table V. and the corresponding visibilities are plotted in fig. 24, the values 
obtained with the arc light being shown by the continuous line and those with 
the N ernst by the dotted curve. The two curves are in very fair agreement 
.to 
20 
30 
40 
SSN 
Fig. 24. 
50 
60 
except on the blue side of the maximum, a region of the spectrum where it is always 
difficult to make concordant measurements of luminosity by the equality of 
brightness method which was employed in this case. These results, together with 
those obtained by previous workers, are shown in fig. 25, where the mean of the values 
obtained with the arc and N ERNST instruments are shown by the full line curve. 
Nutting* and Ives | both used the flicker method of measuring the luminosity. 
Nutting measured the energy distribution directly, while Ives calculated it from the 
energy distribution in the source as obtained by other observers and the measured 
* ‘ Phil. Mag.,’ p. 304, Feb., 1915. 
t ‘Phil. Mag.,’ p. 859, Dec., 1912. 
