=242. Finishing.= On account of the time demanded by other field tasks, it has not been found desirable to make and finish prints in the field. This, with the making of lantern slides, enlargements, etc., may well be turned over to a professional photographer. It is the custom to make a proof of each negative to meet the casual needs that arise in the field. For this purpose, solio “seconds” are used, since they are both cheap and satisfactory. When an urgent demand for a finished print does arise, it is met by using “velox” paper, which can be exposed in the dark room, and then developed and fixed exactly like a plate. Two standard papers for views are “solio” and “platina.” The former gives brown tones, and is used for contrast and brilliancy, hence it is especially good for printing from negatives that have too much detail and too little contrast. “Platina,” on the contrary, yields soft gray tones, and softens contrasts. FORMATION AND SUCCESSION HERBARIA =243. Concept and purpose.= A formation herbarium is a collection of exsiccati, in which the species are arranged with respect to their position in the formation, instead of being grouped in genera and families. Its primary purpose is to furnish a record of the constitution and the structure of a formation or a series of formations. At the same time, it affords the basal material for developing the subject of comparative phytogeography. It is impossible for one ecologist to visit many remote regions, to say nothing of spending a period sufficient for obtaining even a fair knowledge of the vegetation. He can at the best acquire an acquaintance with but few regions at first hand. In consequence, a method that brings a vegetation to him, with its structure carefully wrought out by years of study, is of the highest value. Time, as well as distance, sets a narrow limit to the number of formations which one man can investigate critically in a lifetime. It is no longer possible for a botanist to explore vast regions, and to bring back results which have anything more than a very general value. This fact, far from restricting the comparative study of vegetation, will serve to make it more accurate and systematic. The exact results of numerous resident investigators, expressed in formation herbaria, with the proper series of quadrat maps and photographs, will be worked over by men who are themselves specially acquainted with a particular vegetation. Comparisons will be founded upon a definite basis, and the relationship of various vegetations can then be expressed in precise rather than general terms. It is hardly too sweeping to assert that accurate work in the field of comparative phytogeography can be done only in this fashion. The value of formation herbaria in class work is evident. On account of the limitations of time and distance, classes can touch but few formations, and these at every time except the growing period.