120. Before entering into the details of these applications, I think it necessary to offer some considerations which will enable me to establish some propositions or principles, to which I shall have frequent occasion to refer. I propose to give— 121. 1. Definitions of several expressions applicable to colours and their modifications. 2. The means of representing and defining colours and their modifications by the aid of diagrams. 3. A classification of the harmonies of colours. 4. A view of some arrangements of the primary colours with white, black, and grey. SECTION I. _Definition of the words Tones, Scales, and Hues._ 122. The words _Tones_ and _Hues_ recur continually, both in common language and in that of artists; yet they are not so well defined as to be free from ambiguity, or to be well understood. 123. The word TONE of a colour will be employed exclusively to designate the various modifications which that colour, in its greatest intensity, is capable of receiving from white, which _lowers_ its tone, or of black, which _heightens_ it. 124. The word SCALE will be applied to the assemblage of tones of the same colour, thus modified. The pure colour is the normal tone of the scale, if the normal tone does not belong to a broken or reduced scale—_i.e._, to a scale, of which all the tones are made dull with black (149). 125. The word HUE will be applied exclusively to the modifications which a colour receives from the addition of a small quantity of another. We shall speak, for example, of the _tones_ of the blue scale, the _tones_ of the red scale, &c. We say the _hues_ of blue to designate all the scales whose colours, still remaining blue, yet differ from pure blue; each hue comprehending the tones which constitute a scale more or less allied to the blue scale. 126. I have defined the tones of a colour to be the various modifications, which that colour at its maximum of intensity is capable of receiving from black and white; it must be observed that the condition “maximum of intensity for receiving black,” is absolutely essential to this definition; for if black be added to a tone below the maximum, it would pass into another scale. Artists distinguish colours as _pure, broken, reduced, grey_, or _dull_. 127. Pure colours are those termed _simple_, red, yellow, blue, and those which result from their binary compounds, orange, green, violet, and their _hues_. (150.) _Broken_ colours are the pure colours mixed with black, from the tone of the lightest to the deepest. According to these definitions, it is evident that in all the scales of simple and binary colours, the tones which are above the pure colour are _broken_ tones.