230 OPTICAL PROJECTION 



CHAPTER XV 



PHYSIOLOGICAL DEMONSTBATION 



124. The Projection Microscope, As regards histological 

 detail, the most important aid which can be derived from 

 the lantern is in the form of a projection microscope (see 

 Chapter XIII.). By furnishing members of a class with very 

 inexpensive opera-glasses, a far greater amount of detail can 

 be seen upon the screen, than is possible to the unassisted 

 eye, which, as pointed out at page 191, fails before the 

 microscope, and may be unable to perceive detail really upon 

 the screen. Opera-glasses are thus used in Vienna, and by 

 their aid a single demonstration not only saves much time, 

 but enables the lecturer to use a pointer, and combine the 

 demonstration with a vivd voce explanation, which is found to 

 be more effective than when demonstration under the table 

 microscope is divorced from such explanation. The electric 

 projection microscope, especially, will leave little to be desired 

 as regards by far the larger portion of the histological demon- 

 stration to be done. 



The simpler vital movements, such as circulation in 

 the foot of a frog, or cyclosis in the cells of Vallisneria, as 

 already stated, are also projected easily; even the lime-light 

 making the latter visible to a class of thirty or forty, and the 

 arc light showing it clearly in a larger room. 



125. Mechanical Slides. But the lantern itself is capable 

 of wider application in this class of work. Dealing first with 

 such simple matters as movable mechanical slides, even 

 these are capable of being adapted to serious purposes, as may 

 be shown by the ingenious construction described by Stein 

 for exhibiting in graphic form the main facts of the circulating 

 system. Two lanterns, or nozzles, are necessary, which will 

 be furnished by any form of demonstrator's bi-unial. In one 



