605. When we look at the whole effect of tones of most of the scales made use of in these manufactures, the phenomenon of contrast exaggerates the difference of colour observed between the extreme and the middle tones of the same scale. For instance, in the scale of indigo-blue, applied to silk, the lights are greenish, the browns are tinged violet, while the intermediate tones are blue; but the difference of green and violet at the two extremes is augmented by the effect of contrast. So in the scale of yellow, the light tones appear greener, and the browns redder, than they really are. 606. In speaking of a difference existing between the deep and the light tones of most of the scales of wool and silk, which is exaggerated by contrast, I will add some remarks relative to the gradations the dyer produces. This gradation is very seldom perfect, as the light tones are exactly represented to the eye by the colour taken at its normal tone, reduced by white. Thus, a compound which at the normal tone is pure yellow, or slightly tinged with orange, will, by reduction, produce light tones of a greenish-yellow. An orange-red compound upon silk or wool will yield light tones tinged violet-red. To obtain a correct gradation, we must in most cases add to the weak tones a new coloured material, adapted to neutralize or weaken the defect spoken of. 607. Many of the colouring matters used in painting produce the same result when reduced with white. I do not speak here of changes which may be the effect of chemical action; I allude only to those which result from an attenuation of the coloured material. For example, the normal tone of carmine is a much purer red than its light tones, which are evidently tinged with lilac. Ultramarine, so beautiful in itself, yields light tones which, with respect to the blue rays, appear to reflect more violet rays than the normal tone. In consequence of these facts, it is difficult to colour the chromatic diagram, because many trials must be made to obtain the modification of colour which yields the normal tone of a scale, by the addition of such coloured materials as will render the gradation correct. MEANS AFFORDED BY CONTRAST FOR ASCERTAINING WHETHER THE TONES OF A SCALE OF COLOUR ARE EQUIDISTANT.