I was in hopes at one time that a careful study of cracking and a comparison of the shape and formation of the cracks on different pictures would give me some definite formulæ by which the medium used could be detected, but a very careful examination only gave inadequate results. There appear to be certain thick ridge-like formations in the darker colours especially at the very edges of the draperies, which it is very difficult to imitate in tempera, and while the landscape is most clearly put in quite lightly in tempera, yet the draperies are so different in handling as to give the impression that another medium was used for them. There is also a curious variety in Perugino's pictures as to hatching, especially after 1500, and the hatching which till then only appears in the flesh, is later on to be seen in other parts of the picture. Blending of one colour with another is also to be seen in some of the later pictures to an extent that the earlier pictures never show, as with pure tempera painting the paint "cannot be moved and blended as in oil work," but "it must be put on and left alone till it is dry." I do not pretend to have touched more than the fringe of this subject, but my opinion is that Perugino did many of his pictures wholly in tempera, and that with others he used tempera for the under-painting and oil for the glazes, and that there are also pictures where size has been used as a medium for the draperies when great solidity and heavy cordy outline was desired. The question of oil colours cannot be dismissed in a sentence. Perugino probably learned a good deal as to the use of colours from the Gesuati, who were, as Vasari informs us, well practised in the art of colours, and conversant with glass-painting and enamel work, but the mystery remains as to how it is that the very fugitive colours he used have stood and remained as brilliant as ever. For his Cantor Lectures in 1892,[J] Dr. A. P. Laurie carried out some remarkable experiments with certain fugitive colours and certain oils and balsams, with a view to ascertaining whether any oil varnishes were impervious to moisture, and securely locked up the colours against change. Some of these experiments I have repeated, and I have had some interesting conversation with Dr. Laurie, and I agree with him absolutely that the only vehicles which lock up the colours against moisture and against the action of sulphuretted hydrogen are those composed of balsams. The stickiness of colour ground in balsam to which he alludes can, I have ascertained, be overcome by a few drops of pure linseed oil and a morsel of wax, and the result can then be used easily with a brush. Experimenting with the balsams of the larch and of the silver fir, I have been able to protect verdigris, orpiment, and kermes lake securely, and the experiments have been mainly conducted with verdigris and orpiment as notoriously fugitive colours and yet colours which it is quite clear Perugino used.