cts only 42 per cent. of the light; that is to say, it is
nearly a whole step too low in a decimal scale. Other samples err nearly
as far on the light side of middle value, and further tests prove not
only the varying color sensitiveness of individuals, but detect a
difference between the left and right eye of the same person.

[Illustration: PHOTOMETER.
Back View. Front View.]

The vagaries of color estimate thus disclosed, lead some to seek shelter
in "feeling and inspiration"; but feeling and inspiration are
temperamental, and have nothing to do with the simple facts of vision.
A measured and unchanging scale is as necessary and valuable in the
training of the eye as the musical scale in the discipline of the ear.

It will soon be necessary to talk of the values in each color. We may
distinguish the values on the neutral axis from color values by writing
them N1, N2, N3, N4, N5, N6, N7, N8, N9, N10. Such a scale makes it easy
to foresee the result of mixing light values with dark ones. Any two
gray values unite to form a gray midway between them. Thus N4 and N6
being equally above and below the centre, unite to form N5, as will also
N7 and N3, N8 and N2, or N9 and N1. But N9 and N3 will unite to form N6,
which is midway between 6 and 9.

[Illustration: Vertical Section through light openings.

PARTS.

_C_, CABINET, with sample-holder (H) and mirror (M), which may be
removed and stored to left of dial (D) when instrument is closed
for transportation.
_D_, DIAL: records color values in terms of standard white (100),
the opposite end of the scale being absolute blackness (0).
_E_, EYE-PIECE: to shield eye and sample from extraneous light while
color determinations are being made. Fatigue of retina should be
avoided.
_G_, GEAR: actuates cat's-eye shutter, which controls amount of
light admitted to right half of instrument. Its shaft carries
index-hand over dial.
_H_, FIELD-HOLDER: retains sample and standard white in same plane,
and isolates them. Is hinged upon lower edge, and secured by pivot
clamp.
_M_, MIRROR: permits observation of the isolated halves of the
holder, bearing standard white and the color to be measured. Should
be clean and free from dust on both sides of central partition.
_S_, DIFFUSING SCREEN, placed over front apertures, to evenly
distribute the light.]

(66) When this numbered scale of values is familiar, it serves not only
to describe ligh