(17) This forms an inclined circuit, crossing the equator at opposite points, and suggests the ecliptic or the rings of Saturn (see outside cover). A pale rainbow would describe a slanting circuit nearer white, and a dimmer one would fall within the sphere, while an intensely brilliant spectrum projects far beyond the surface of the sphere, so greatly is the chroma of its hues in excess of the common pigments with which we work out our problems. (18) At the outset it is well to recognize the place of the spectrum in this system, not only because it is the established basis of scientific study, but especially because the invariable order assumed by its hues is the only stable hint which Nature affords us in her infinite color play. (19) All our color sensations are included in the color solid. None are left out by its scales of hue, value, and chroma. Indeed, the imagination is led to conceive and locate still purer colors than any we now possess. Such increased degrees of color sensation can be named, and clearly conveyed by symbols to another person as soon as the system is comprehended. APPENDIX TO CHAPTER I. +Misnomers for Color.+ The Century Dictionary helps an intelligent study of color by its clear definitions and cross-references to HUE, VALUE, and CHROMA,--leaving no excuse for those who would confuse these three qualities or treat a degree of any quality as the quality itself. Obscure statements were frequent in text-books before these new definitions appeared. Thus the term “shade” should be applied only to darkened values, and not to hues or chromas. Yet one writer says, “This yellow shades into green,” which is certainly a change of hue, and then speaks of “a brighter shade” in spite of his evident intention to suggest a stronger chroma, which is neither a shade nor brighter luminosity. Children gain wrong notions of “tint and shade” from the so-called standard colors shown to them, which present “tints” of red and blue much darker than the “shades” of yellow. This is bewildering, and, like their elders, they soon drop into the loose habit of calling any degree of color-strength or color-light a “shade.” _Value_ is a better term to describe the light which color reflects to the eye, and all color values, light or dark, are measured by the _value-scale_. “Tone” is used in a confusing way to mean different things. Thus in the same sentence we see it refers to a single touch of the brush,--which is not a tone, but a paint spot,--and then we read that the “tone of the canvas is golden.” This cannot mean that each paint spot is the color of gold, but is intended to suggest that the various objects depicted seem enveloped in a yellow atmosphere. Tone is, in fact, a musical term appropriate to sound, but out of place in color. It seems better to call the brush touch a _color-spot_: then the result of an harmonious relation between all the spots is _color-envelope_, or, as in Rood, “the chromatic composition.”