HIRIvFKINGBNCE. 
109 
seems to be the best standard of reference available. Ives has proposed 
as a standard the light emitted from a black body at 5000, but this also is 
not entirely satisfactory for many reasons. In determining the visual 
intensity of a light source for different parts of the spectrum, it has been 
found by experience that "the ratio of light to energy (the visual sensi- 
bility of the eye) varies with the wave-length and, in the range i to 10 meter 
candles (roughly), with the intensity as well. This ratio depends upon the 
sensibility of the eye, the so-called visibility of radiation."* Visual sensi- 
bility curves for white light have been determined for high intensities and 
for low intensities and found not to agree precisely. Both curves have a 
single maximum which is located for low intensities at about 510/1/1 and for 
high intensities at about 545 /z/z. At low intensities, below i meter candle 
down to the threshold value, the vision is largely achromatic (rod vision) 
while for intensities above 10 meter candles the vision is chiefly chromatic 
(cone vision.) Between i to 10 meter candles both the rods and cones of the 
retina of the eye are active and the maximum of the visibility curve lies 
90 
80 
70 
50 
40 
30 
20 
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50 500 540 550 560 570 580 600 ; 650 
blue J green j yellow '. orange j red 
wave lengths 
FIG. 69. Curves showing percentage intensity (referred to that of brightest part of 
spectrum at 545 MM) of transmitted light for path-differences 540, 550, 560, 570, and 
580 pp. The solid-line curve is the luminosity curve. The scale of the dotted intensity 
curves is ten times that of the solid-line luminosity curve. Thus for a path-difference 
550 MM. the visual brightness of the color X = 500 MM is 3.6 per cent of that of the brightest 
color (545 MM) in the daylight used as source of illumination. For the path-difference 
550 MM the amounts of light transmitted from the red and blue ends of the spectrum are 
about equal (2.87 per cent in the red and 2.96 per cent in the blue end from the original 
light, the total amount of light transmitted for this path-difference being 5.83 per cent of 
the original source). 
between 510 and 545 /z/z. The actual energy distribution of the visible 
solar spectrum for midday sun has often been measured and found to have 
a maximum at about 520 /z /z, the energy curve sloping gradually to about 
75 per cent of the maximum at 400 /z/z and about 80 per cent at 700 ti/z.f 
From the energy and visual sensibility curves the curve of actual light 
can be found directly (the visibility being the ratio of the light to the energy 
.. L 
V= E t 
= E.V). This is indicated by the full line curve of Fig. 69, 
*Scc P. G. Nutting, circular, Bureau of Standards, No. 28, 7. 1911. 
tNutting, P. G., Circular. U. S. Bureau of Standards, No. 28, 6, 191 1 ; Abbot and Fowle. Annals Astroph. 
Observatory, 2, 105, 1905. 
