"Every opaque surface partakes of the colour of the intervening transparent medium, according to the density of such medium and the distance between the eye and the object. "The medium is of two kinds; either it has a surface, like water, &c., or it is without a common surface, like the air."[21] In the observations on trees and plants more points of resemblance might be quoted; the passages corresponding with Goethe's views are much more numerous. It is remarkable that Leonardo, in opposition, it seems to some authorities,[22] agrees with Aristotle in reckoning black and white as colours, placing them at the beginning and end of the scale.[23] Like Aristotle, again, he frequently makes use of the term black, for obscurity; he even goes further, for he seems to consider that blue may be produced by the actual mixture of black and white, provided they are pure.[24] The ancient author, however, explains himself on this point as follows--"We must not attempt to make our observations on these effects by mixing colours as painters mix them, but by remarking the appearances as produced by the rays of light mingling with each other."[25] When we consider that Leonardo's Treatise professes to embrace the subject of imitation in painting, and that Aristotle's briefly examines the physical nature and appearance of colours, it must be admitted that the latter sustains the above comparison with advantage; and it is somewhat extraordinary that observations indicating so refined a knowledge of nature, as regards the picturesque, should not have been taken into the account, for such appears to be the fact, in the various opinions and conjectures that have been expressed from time to time on the painting of the Greeks. The treatise in question must have been written when Apelles painted, or immediately before; and as a proof that Aristotle's remarks on the effect of semi-transparent mediums were not lost on the artists of his time, the following passage from Pliny is subjoined, for, though it is well known, it acquires additional interest from the foregoing extracts. "He (Apelles) passed a dark colour over his pictures when finished, so thin that it increased the splendour of the tints, while it protected the surface from dust and dirt: it could only be seen on looking into the picture. The effect of this operation, judiciously managed, was to prevent the colours from being too glaring, and to give the spectator the impression of looking through a transparent crystal. At the same time it seemed almost imperceptibly to add a certain dignity of tone to colours that were too florid." "This," says Reynolds, "is a true and artist-like description of glazing or scumbling, such as was practised by Titian and the rest of the Venetian painters."