ANATOMICAL TECHNIQUE 467 



Europe, but these are now made in good variety and of excellent 

 quality by the Cramer Dry Plate Company, of St. Louis, Missouri. 

 All the types of plates described above may be purchased from the 

 Cramer Company. Since ordinary photographic supply houses 

 often do not carry in stock the special plates needed for photo- 

 micrographic and other scientific purposes, the Cramer Company 

 undertakes to furnish these direct from its factory in St. Louis. 



The making of lantern slides is an activity which may often be 

 pursued profitably in larger botanical establishments in univer- 

 sities. The most convenient method of making such slides is by 

 placing the lantern plate directly behind a negative of suitable 

 size, namely, 3! by 4^ inches. A few seconds' exposure to a light 

 sheltered by ground glass in the dark room (the time depending 

 on the character of the negative) suffices. Development of lan- 

 tern slides is effected by means of a special developer to be men- 

 tioned in a subsequent paragraph. The slides must be covered 

 with a mask which can be made by cutting out black paper by 

 means of a wheel paper-cutter and appropriate metal forms, obtain- 

 able at photographic supply stores. Masks may also be purchased 

 with the various forms and sizes of opening necessitated by the 

 different kinds of pictures. After the mask is applied the lantern 

 slide is protected by a thin cover-glass held in position by black 

 paper binding strips which may be procured in any photographic 

 establishment. American and Continental European lantern slides 

 are 35 by 4 inches in dimension. The English slide is 3! inches 

 square. The storing of lantern slides is always a problem when 

 their number becomes large, and many filing cabinets for this pur- 

 pose are on the market. The essential thing, however, is to have 

 numbers on the slides and a catalogue, so that they can readily 

 be selected for use and easily returned to their places. The com- 

 plexity of the catalogue will of course depend upon the tastes and 

 needs of the individual and the extent of his budget. 



The development of the various plates described in the fore- 

 going paragraph is effected by reagents which reduce to metallic 

 silver the parts of the plate exposed to the light. Developers are 

 legion, but two or three seem to answer every practical purpose. 

 For making negatives pyrogallol developer is on the whole most 



