No exception can be taken against Dr. Aitken's argument in your number for Oct. 12. The colours of the substances he experimented on could not be regarded as simple. But he does not consider how loosely all names of colours must be applied in common language. The colours of most blue pigments, especially in thin washes, no doubt contain a large proportion of green. But let the colour of the blue salvia, or that of the pigment called French blue or ultramarine (often given as the best example of true blue) be tested in conjunction with the purest yellows (even with the almost greenish yellow of the pigment called lemon-yellow) and the two will be found perfectly complementary. This is the colour of Newton's indigo rays, which he himself in his colour circle put opposite to his yellow. In fact, in good English, not only sea-greenish blues, like the colour of Newton's blue part of the spectrum, or that of the pigment called azure or cæruleum, but even the colour of the violet itself, is properly called blue. Witness Milton's “beds of violets blue.” The violet of the spectrum is in truth little more than a pure blue diluted with white by reason of the fluorescence of the retina, as recent researches have shown. (See J. J. Müller's paper in Poggendorff's Annalen, March and April last.) I must, therefore, protest against substituting a fanciful term like violet for the good English blue, as the designation of a simple colour-sensation. It is hard enough to make artists believe that yellow is not a simple colour. To tell them the same of its complementary blue would add to their disgust, and not unreasonably.