The last three genera of colours comprehend in an orderly gradation all those which are _positive_ or definite; and the three colours of each genus, united or compounded in such subordination that neither of them predominates to the eye, constitute the _negative_ or neutral colours, of which _black_ and _white_ have been stated to be the opposed extremes, and _greys_ their intermediates. Thus black and white are constituted of, and comprise latently, the principles of all colours, and accompany them in their depth and brilliancy as shade and light. _Semi-neutral Colours_ belong to a class of which _brown_, _marrone_, and _gray_ may be considered types. They are so called, because they comprehend all the combinations of the primary, secondary, and tertiary colours, with the neutral _black_. Of the various combinations of black, those in which yellow, orange, or citrine predominates, have obtained the name of brown, &c. A second class in which the compounds of black are of a predominant red, purple, or russet hue, comprises marrone, chocolate, &c. And a third class, in which the combinations of black have a predominating hue of blue, green, or olive, includes gray, slate, &c. While treating of the classes of colours, it may not be out of place to note here the difference between gray as spelt with an _a_, and grey as spelt with an _e_, the two names being occasionally confounded. _Gray_ is semi-neutral, and denotes a class of cool cinereous colours, faint of hue; whence we have blue grays, olive grays, green grays, purple grays, and grays of all hues in which blue predominates; but no yellow or red grays, the predominance of such hues carrying the compounds into the classes of brown and marrone, of which gray is the natural opposite. _Grey_ is neutral, and is composed of or can be resolved into black and white alone, from a mixture of which two colours it springs in an infinite series. It must be observed that each colour may comprehend an indefinite series of _shades_ between the extremes of light and dark, as each compound colour also may comprise a similar series of _hues_ between the extremes of the colours composing it. And as the relations of colours have been deduced regularly, from white or light to black or shade; so the same may be done, inversely, from black to white. On this plan the tertiaries, olive, russet, and citrine, take the place of the primaries, blue, red, and yellow; while the secondaries still retain their intermediate station and relation to both. Thus, _russet and olive_ compose or unite in _dark purple_; _citrine and olive_ in _dark green_; _russet and citrine_ in _dark orange_. The tertiaries have, therefore, the same order of relation to black that the primaries have to white; and we have black primaries, secondaries, and tertiaries, inversely, as we have white primaries, secondaries, and tertiaries, directly. In other words, we have light and dark colours in all classes. CHAPTER IV.