(64) There remains the question, What will happen if we mix, not two neighbors, nor two opposites, but _a pair of hues once removed in the circle_, such as red and green? A line joining this pair does not pass through the neutral centre, but to one side nearer yellow, which shows that this mixture falls between neutral gray and yellow, partaking somewhat of each. In the same way a line joining yellow and blue shows that their mixture contains both green and gray. Indeed, a line joining any two colors in the circuit may be said to describe their union. A radius crossing this line passes to some hue on the circumference, and describes by its intersection with the first line the chroma of the color made by a mixture of the two original colors. Red & Green make Yellow-gray } Yellow Blue Green-gray } Each pair unites in a _colored_ Green Purple Blue-gray } gray, which is an intermediate hue Blue Red Purple-gray } of weak chroma. 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 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.) [Illustration: 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 gray 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)[19] is shown on the next page. It measures the relative amount of light which the eye receives from any source, and so enables us to make a scale with any number of regular steps. The principle on which it acts is very simple. [Footnote 19: 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.]