TO THE TEACHER 13 



have a diameter of J^ o inch. Six red corpuscles, 

 lying flat in a row with their edges touching, would 

 about span the cut end of the hair. It would take half 

 a dozen bacteria of average size, laid end to end, to reach 

 across the disc of a single red corpuscle. When the micro- 

 scope is used, it is a convenience to bear in mind the real 

 size of the visible field. For a magnification of 100 diame- 

 ters this is usually about J^o inch; for 500 diameters 

 it is about J^ oo- 



At the same time that the pupil is required to think in 

 terms of microscopic measurements he is introduced to 

 molecular and atomic ideas. He is in danger of failing 

 to realize how great is the difference in scale between the 

 two. He must not be allowed to think that the micro- 

 scope can bring the molecules of a true solution to visi- 

 bility. The effort must be to explain to him that this is 

 utterly beyond accomplishment. The step from the 

 gross structure to the cellular is short indeed compared 

 with the further transition to the realm of molecules. 



A word about helps in teaching. A first-class manikin 

 is a treasure greatly to be desired. Its superiority to 

 charts and diagrams lies in its solidity, its representation 

 of three dimensions in the only convincing way that can 

 be employed. However, a good manikin is expensive. 

 Those which are flat and made to unfold, layer after 

 layer, are not realistic enough to be preferred to charts, 

 and charts in turn may be dispensed with for most pur- 

 poses if the teacher will cultivate blackboard drawing. 

 The great advantage of chalk lies in the fact that what- 

 ever feature is under discussion may be made to stand 

 out and no details which are not helpful at the moment 

 need be in view. It is much easier to follow the exposi- 

 tion when the structures referred to appear successively 

 than when they are all presented at once as in a com- 

 pleted figure. 



The teacher of physiology must take a middle course in 

 the endeavor to correct two extreme tendencies commonly 

 manifested by immature students. The very same 



