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photometric curves, they serve to show the comparative
intensities of the various wave-lengths present.
In Fig.5 the spectrogram at A is that {or acetylene
and at B for noon sunlight. These are given as a
basis of comparison in terms of which to judge the
distribution of radiation in the spectra o{ flame arcs
shown at C, D, E and F; C being that for a white
flame arc, D the blue flame arc, E the yellow flame
and F the red flame. It will be noted that the spectrum
of the white flame arc resembles that of sunlight
except that there is an appreciable enhancement
of the radiation in the blue between 400 and
520 mp. It is difficult to compare the spectrum of
the yellow and red flame arcs with that of sunlight,
but it is evident that there is a general predominance
o{ intense lines in the long wave-length region between
540 and 700 mp.
Fig. 6 shows the distribution o{ energy in the
radiation emitted by the Cooper-Hewitt mercury
vapor lamp. This is a source of the line type, energy
being emitted at the wave-length indicated by
the heavy vertical lines, the relative intensity emitted
at each wave-length being indicated by the height
of the line.
Now, having dealt with the characteristics of the
radiation emitted by various light sources, we shall
pass on to a consideration of the human eye as a
sense organ responding to a stimulation by the ac-
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WAVE'LENGTH. rN mt
FIGUR,E
Diagram showing the relative amount of energy radiated at
diferent wave-lengths by the Cooper-Hewitt mercurv
vapor lamp.
tion of radiant energy. The sensation produced
when radiation falls upon the retina is re{erred to
in general as light. This sensation has three distinct
attributes: brilliance, hue, and saturation.
Brilliance is that attribute oI any color in respect
to which it may be classed as equivalent to some
member of the series of grays ranging between black
and white.
Hue is that attribute of certain colors in respect
to which they difier characteristically from a gray
of the same brilliance and which permits them to be
classed as reddish, yellowish, greenish, or bluish,
etc.
Saturation is that attribute of all colors possessing
a hue which determines the degree of diflerence
{rom a gray of the same brilliance.
VI
An object in the visual field is visible by virtue of
the contrast between it and its immediate surroundings
or background. This contrast may be due to
a difierence in hue (hue contrast), to a difierence
in saturation (saturation contrast), or to a difierence
in brilliance (brilliance contrast). In the case
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FIGURE
VII
Visibility cutve for high brightness levels.
of a photographic reproduction such as a print or
image projected on to a screen, since all hue contrast,
and consequently all saturation contrast, is
absent, visibility of object detail depends entirely
upon the existence of a brilliance contrast. It follows,
therefore, that the reproduction of detail by
the photographic processes must be accomplished by
reproducing as a brilliance contrast that contrast
which in the object may be due to either a contrast
of hue, saturation, or brilliance. This being the
case, the visual function giving the relation between
wave-length of radiation and the brilliance factor
of the resulting sensation is of prime itnportance.
This relationship is known as the visibility function
and its form is obiained by measuring the magnitude
of the brilliance attribute of the sensation produced
when the same radiation intensity o{ various
wave-lengths acts upon the retina. In Fig' 7 this
visibility curve is shown. It is obtained by plotting
as ordinates the magnitude o{ the brilliance sensation
produced by constant energy o{ the various
wave-lengths as indicated along the bottom of the
figure. Thus we see that for a fixed amount of
energy the maximum sensation is produced by wavelength
554 mp. From this point the curve falls
rapidly for both shorter and longer wave-lengths,
approaching zero at about 400 m/, on the one
hand and 700 mp on the other. Thus it is evident
that the visible spectrum is limited {or all practical
purposes by wave-length 400 mpr, at the blue end of
the spectrum, and by wave-length 700 mp at the red
end. A knowledge of this relationship is essential
for the computation of the visual effect produced
by radiation of any specified spectral composition,
and is of fundamental importance in all problems
dealing with the reproduction o{ visual tone values
by the photographic processes. Used in conjunction
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