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the chroma of the color made by a mixture of the two original colors.

Red & Green make Yellow-gray
Each pair unites in a colored gray, which is an intermediate hue of weak chroma.
Yellow Blue Green-gray
Green Purple Blue-gray
Blue Red Purple-gray
Purple Yellow Red-gray

Mixture of white and black: a scale of grays.

(65) So far we have thought only of the plane of the equator, with its circle of middle hues in ten steps, and studied their mixture An image should appear at this position in the text. by drawing lines to join them. Now let us start at the neutral centre, and think upward to white and downward to black (Fig. 9.)

This vertical line is the neutral axis joining the poles of white and black, which represent the opposites of light and darkness. Middle 4) pray is half-way between. If black is called 0, and white is 10, then the middle point is 5, with 6, 7, 8, and 9 above, while 4, 3, 2, and 1 are below, thus making a vertical scale of grays from black to white (Chapter II., paragraph 25).

If left to personal preference, an estimate of middle value will vary with each individual who attempts to make it. This appears in the neutral scales already published for schools, and students who depend upon them, discover a variation of over 10 per cent. in the selection of middle gray. Since this value scale underlies all color work, it needs accurate adjustment by scientific means, as in scales of sound, of length, of weight, or of temperature.

A Photometer (photo, light, and meter, a measure)[1] is shown

  1. Adopted in Course on Optical Measurements at the Massachusetts Institute of Technology. Instruments have also been made for the Harvard Medical School, the Treasury Department in Washington, and various private laboratories, and commercial industries.