The same is true of lantern-slides to an even greater degree, for slides are practically never used except in groups. As a collection of slides may be grouped in scores of ways, it is better to file them in some order that will admit of quick selection, than to form groups arbitrarily at the outset and keep these together. A slide in such a group is practically withdrawn from the possibility of assemblage in some other group. For instance, a view of Michael Angelo’s “Moses” might find a place in a group to illustrate a talk on Michael Angelo, or Renaissance Sculpture, or The Art Treasures of Rome, or Old Testament Worthies, or any one of a dozen others. If we place it arbitrarily in any one of these and keep the group together, we shall of course spare ourselves a little trouble if anyone wants that particular assemblage of slides, but we shall not only make it more difficult to assemble the other groups, but practically put them out of the running. Several years ago we had a valuable gift of a collection of slides illustrating phases of city-planning, given by the Civic League of our city. They included many foreign views now difficult or impossible to obtain. The donors had assembled them in groups to go with lectures prepared in advance and we maintained this arrangement for a time, although it was not in accord with our general plan. But we soon found that persons who asked for slides on London or Munich or Milan were missing some of our best material, simply because we could not always remember to look through the city-planning groups for something that might be there. Consequently we broke up these groups and distributed their slides to the proper places in our file, which is in trays arranged precisely as if the slides were catalogue cards, with proper guides and cross-references on cardboard slips. We have memoranda of the slides that belong in each lecture group and these can be quickly assembled if wanted. Of course we allow the public to go directly to the trays if they desire and assemble for themselves any group that they choose. This is all borderland material between library and museum. There is much of it analogous to the lantern slide that libraries have not taken up yet, but that they might handle to good advantage. I do not see why we should not, for instance, circulate microscope slides or photographic negatives. Stereoscopic pictures are now commonly handled by libraries owing to skilful and perfectly legitimate exploitation.