Page:Popular Science Monthly Volume 61.djvu/162

156 it seems that either the stereopticon, or wall pictures large enough to be seen by the class as a whole, are most desirable. The instructor may thus explain to a class in a few minutes what would under other conditions require hours to make clear to them. This mode of procedure is as useful with pictures of the infinitesimal organisms and material in nature as with views of miles of landscape concerned with the geography of the globe.

Take for example the little daphina, shown in Fig. 4, magnified 150 diameters. This when thrown from a slide on an eightfoot screen makes the original magnified to nearly six thousand diameters. A class of students may look first at a tiny speck in a glass of clear water, which is perhaps one third the size of a pin-head, apparently without definite form, and of no consequence, but which, when seen enlarged, is shown to possess all the organs of a living animal. And further to illustrate how very tiny a particle of matter may become and yet be a mass as distinguished from molecules and atoms, the student has only to note such an illustration; for he sees that this microscopic animal is a mass as truly as is the elephant. In the study of rocks, of plants, of animals, even in a most elementary way, some very instructive lessons concerning their minute structure and how it concerns their outward forms and functions may be learned by such pictures.

Of course, all schools are not so situated as to be able to use a lantern in this way, though many have felt the importance of its use and of making a place for it. With the cheap forms of apparatus on the market at present and the readiness with which electricity or acetylene gas is obtained for an illuminant, the lantern should find its proper place in the class-room. Various forms of apparatus are also accessible for the projection of the microscopic slides directly, but in general, they give more trouble in handling to the average person than they are worth. The lantern slide is infinitely more satisfactory for general illustrative teaching. Another and in some eases better method for illustrating a subject is by the use of large pictures which can be hung up before a class and readily explained. Where classes are not too large, a photograph 20″ x 24″ in size is large enough, and there is then no necessity for a darkened room as with the lantern, and the picture may be considered in just the proper place in the course of a talk on the subject. These pictures are simple enlargements on bromide or velox paper from the original microscopic negative and cost but little more than lantern slides.

Since the scientific knowledge structure is continually building upon the foundation of accepted results of earlier investigators, it may be said in conclusion that photomicrography serves a double purpose. First, itenables the scientific investigator to determine accurately a knowledge of the minute physical structure of matter, and secondly, it provides a means of placing such information before others in a comprehensive manner.