_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 light and dark grays, but the value of colors which are at the same level in the scale. Thus R7 (popularly called a tint of red) is neither lighter nor darker than the gray of N7. A numeral written above to the right always indicates _value_, whether of a gray or a color, so that R1, R2, R3, R4, R5, R6, R7, R8, R9, describes a regular scale of red values from black to white, while G1, G2, G3, etc., is a scale of green values. (67) This matter of a notation for colors will be more fully worked out in Chapter VI., but the letters and numerals already described greatly simplify what we are about to consider in the mixture and balance of colors. +Mixture of light hues with dark hues.+ (68) Now that we are supplied with a decimal scale of grays, represented by divisions of the neutral axis (N1, N2, etc.), and a corresponding decimal scale of value for each of the ten hues ranged about the equator (R1, R2,-- YR1, YR2,-- Y1, Y2,-- GY1, GY2,-- and so on), traced by ten equidistant meridians from black to white, it is not difficult to foresee what the mixture of any two colors will produce, whether they are of the same level of value, as in the colors of the equator already considered, or whether they are of different levels. [Illustration: Fig. 10.] (69) For instance, let us mix a light yellow (Y7) with a dark red (R3). They are neighbors in hue, but well removed in value. A line joining them centres at YR5. This describes the result of their mixture,--a value intermediate between 7 and 3, with a hue intermediate between R and Y. It is a yellow-red of middle value, popularly called “dark orange.” But, while this term “dark orange” rarely means the same color to three different people, these measured scales give to YR5 an unmistakable meaning, just as the musical scale gives an unmistakable significance to the notes of its score.