122. We must not, however, pass entirely without notice the two chapters on the preparation of oils, and on the oleo-resinous vehicles, though to the general reader the recipes contained in them are of little interest; and in the absence of all expression of opinion on the part of Mr. Eastlake as to their comparative excellence, even to the artist, their immediate utility appears somewhat doubtful. One circumstance, however, is remarkable in all, the care taken by the great painters, without exception, to avoid the yellowing of their oil. Perfect and stable clearness is the ultimate aim of all the processes described (many of them troublesome and tedious in the extreme): and the effect of the altered oil is of course most dreaded on pale and cold colors. Thus Philippe Nunez tells us how to purify linseed oil "for white and blues;" and Pacheco, "el de linaza no me quele mal: aunque ai quien diga que no a de ver el Azul ni el Blanco este Azeite."[17] De Mayerne recommends poppy oil "for painting white, blue, and similar colors, so that they shall not yellow;" and in another place, "for air-tints and blue;"--while the inclination to green is noticed as an imperfection in hempseed oil: so Vasari--speaking of linseed-oil in contemporary practice--"benche il noce e meglio, perche ingialla meno." The Italians generally mixed an essential oil with their delicate tints, including flesh tints (p. 431). Extraordinary methods were used by the Flemish painters to protect their blues; they were sometimes painted with size, and varnished; sometimes strewed in powder on fresh white-lead (p. 456). Leonardo gives a careful recipe for preventing the change of color in nut oil, supposing it to be owing to neglect in removing the skin of the nut. His words, given at p. 321, are incorrectly translated: "una certa bucciolina," is not a husk or rind--but "a thin skin," meaning the white membranous covering of the nut itself, of which it is almost impossible to detach all the inner laminae. This, "che tiene della natura del mallo," Leonardo supposes to give the expressed oil its property of forming a _skin_ at the surface. 123. We think these passages interesting, because they are entirely opposed to the modern ideas of the desirableness of yellow lights and green blues, which have been introduced chiefly by the study of altered pictures. The anxiety of Rubens, expressed in various letters, quoted at p. 516, lest any of his whites should have become yellow, and his request that his pictures might be exposed to the sun to remedy the defect, if it occurred, are conclusive on this subject, as far as regards the feeling of the Flemish painters: we shall presently see that the _coolness_ of their light was an essential part of their scheme of color.