The general survey I am about to treat you to is almost entirely based on the information supplied by librarians, teachers and curators of museums and galleries. May I say now how grateful I am to those who have been so generous with their time in telling me most of what I know about the subject. I regard my role this evening as essentially one of disseminating the knowledge of others which in this field does not always circulate as freely as it might.

What has rather surprised me while preparing this talk is how quite important material in one centre is unknown in another. My talk will serve some use, therefore, if it helps to rectify this situation.