156 POPULAR SCIENCE MONTHLY. 



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 con- 

 ditions 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 geog- 

 raphy of the globe. 



Take for example the little daphina, shown in Fig. 4, magni- 

 fied 150 diameters. This when thrown from a slide on an eight- 

 foot 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, ap- 

 parently 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 out- 

 ward 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 mak- 

 ing 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 ob- 

 tained for an illuminant, the lantern should find its proper place in the 

 class-room. Various forms of apparatus are also accessible for the pro- 

 jection of the microscopic slides directly, but in general, they give more 

 trouble in handling to the average person than they are worth. The lan- 

 tern 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"x24" 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, iten- 

 ables 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. 



