Of the colour of painting grounds, we have it on the authority of Mrs. Merrifield, that "One cause of the purity and beauty of the colours in ancient painting is the care with which the grounds were prepared. When these were not of gold, they were invariably white; and we find, from the work before us, that no pains were spared to preserve them, pure, clean, and bright; for on this the success of the painting appeared to depend. 'All they,' says De Piles, the commentator on Du Fresnoy's Art of Painting, 'who have coloured well, have had another maxim to maintain their colours fresh and flourishing, which was to make use of white grounds, upon which they painted, and oftentimes at the first stroke, without retouching anything, and without employing new colours. Rubens always used this; and I have seen pictures from the hand of this great person, painted up at once which were of a wonderful vivacity. The reason why they made use of those kinds of grounds is because white not only preserves a brightness under the transparency of colours, which hinders the air from altering the whiteness of the ground, but also repairs the injury which they receive from the air, so that the ground and colours assist and preserve each other. It is for this reason that glazed colours have a vivacity which can never be imitated by the most lively and most brilliant colours; because, according to the common way, the different tints are laid on each in its place, one after another. So true is it that white , with other strong colours, with which we paint at once what we intend to glaze, gives life, spirit, and lustre to the work. The ancients most certainly found that white grounds were much the best; for although they were conscious of the injury which their eyes received from that colour, yet they did not forbear the use of it.'" In his tenth book 'Of the Use of the Parts,' van Galen says: "Painters, when they work upon white grounds, place before them dark colours and others, mixed with blue and green, to refresh their eyes; because white is a glaring colour, which wearies and pains the sight more than any other.",