(_b_) The second aspect of externality is the relation it occupies directly in the various material which art employs in the various media of its presentations. The unity here consists in the clear definition and homogeneousness of the material, which ought not to deviate in the direction of a vague characterization and mere confusion, or, speaking generally, give us the impression of dirtiness. This determinacy is also entirely dependent upon the spatial condition, that is to say, upon the purity of its delineation, the distinctness of its rectilinear and circular lines, and so on, no less than upon a consistent definition of Time such as we find in the accurate measure of the musical beat. It depends furthermore on the translucency both of specific tones and colours. In a good picture we shall find that there is nothing unclean or "dirty"[376] in the colours employed, but everything is clear and asserts itself openly for what it is. The directness of its purity is, in fact, that which constitutes the lovely impression of colour upon our sense; and those colours which are most direct in their simplicity, such as a yellow which has no dash of green in it, or a red that is wholly independent of blue or yellow, produce the most emphatic effect. On the other hand, it is obviously more difficult to maintain a harmony of the whole when colours are thus contrasted in all their pristine simplicity. Yet in despite of this these essentially simple colours form the foundation of every true colour scheme, and although it may be impossible to dispense with a considerable use of their compounds, even these should not be allowed to appear in one dead and dull interfusion, but with their simple and luminous derivations shining through them[377]; otherwise instead of the clear lambency of colour we shall get nothing but a muddy residuum. We shall find that the same thing is necessary in the _timbre_ of musical tone. In the case of strings whether of metal or catgut it is the vibration of the material, and, moreover, of a material of definite tension and length, which educes the musical note. If the tension is insufficient, or the length of the string which is struck is not the right one, the tone inevitably loses its clearness of definition, rings false, as we say, owing to the interfusion with it of other tones[378]. We have the same kind of result when a purely mechanical fretting or scraping is suffered to interfere with the purity of the vibratory motion, and so to render the emitted sound confused and harsh. In the same way, it is of the first importance to the art of singing that the human voice should be produced from throat and chest freely and with purest intonation; the voice should be heard without the least indication of its organic instrument, or, as in the case of hoarseness, with an obstructive accompaniment the singer fails to repress. We may conclude, then, that this translucency and purity,