A rectangular box, divided by a central partition into halves, has symmetrical openings in the front walls, which permit the light to reach two white fields placed upon the back walls. If one looks in through the observation tube, both halves are seen to be exactly alike, and the white fields equally illuminated. A valve is then fitted to one of the front openings, so that the light in that half of the photometer may be gradually diminished. Its white field is thus darkened by measured degrees, and becomes black when all light is excluded by the closed valve. While this darkening process goes on in one-half of the instrument, the white field in the other half does not change, and, looking into the eyepiece, the observer sees each step contrasted with the original white. One-half is thus said to be _variable_ because of its valve, and the other side is said to be _fixed_. A dial connected with the valve has a hand moving over it to show how much light is admitted to the field in the variable half. Let us now test one of these personal decisions about middle value. A sample replaces the white field in the fixed half, and by means of the valve, the white field in the variable half is alternately darkened and lightened, until it matches the sample and the eye sees no difference in the two. The dial then discloses the fact that this supposedly MIDDLE VALUE reflects 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.