582 THE POPULAR SCIENCE MONTHLY. 



used, by far tlie greater part of any class will, from mere force of 

 babit, commit the contents, and then imagine tbey see everytbing 

 mentioned in tbe books and notbing more. After tbey bave been 

 trained to observe, they may be allowed to consult books, but not be- 

 fore. What is true of books is true of lectures on objects taught in 

 tbe laboratory. Tbe students always wish to bave tbe lecture first 

 and see the object afterward. It seems to them to lighten the work, 

 but tbey fail to recognize, what is evident to the instructor, that they 

 are not learning so much or so well. 



Again, few stuflents have any proper conception of solid bodies, 

 and, to train them on this point, nothing is so good as some opaque 

 body which has to be studied by microscopic sections. For this pur- 

 pose I use pieces of pine-wood which are given to the class early in 

 the term, just as soon as they bave acquired a little facility in the use 

 of the microscope. A piece large enough to show the annual rings is 

 given to each student, who, by looking at the rings, can tell from what 

 part of the trunk his piece came. After some simple directions about 

 cutting, the student is told to make sections in three directions : at 

 right angles to the trunk, and in the directions of the radius and tan- 

 gent, and in the order named. After they have made and drawn the 

 first section, if asked what they think is tbe structure of the wood, 

 almost all of them will at once say that it is composed of square cells. 

 If one asks what they mean by square cells, they say cells shaped lik$ 

 dice. In classes of from thirty to forty persons, I have never found 

 more than four or five students — in one class there was only one — who 

 knew enough to say that they could not tell what the structure of the 

 wood was until tbey had seen sections in other directions. The cross- 

 section made, they proceed to the radial section. Having already 

 made up their minds from the cross-section that the wood is formed 

 of cubical cells, the radial section, with its long tubes showing the 

 peculiar disk-like markings of coniferous wood on tbe walls, utterly 

 confounds them ; and it requires considerable time before they give 

 up the attempt to make what they see in the radial section agree with 

 the cubical cells which exist only in their own imaginations, and 

 realize that it is only by mentally combining the transverse and radial 

 sections that they can arrive at any correct conception of the structure 

 of the wood. Finall}^, the disk-like markings are to be explained. 

 After trying ineffectually to pass them off as nuclei, vacuoles, or other 

 structures which they have heard are to be found in vegetable cells, 

 they are finally induced to see whether they can not find any traces of 

 them in the other section, and so, slowly, they make out their real 

 nature. 



No work which I ever have to do as an instructor is so utterly 

 dreary as that of forcing students to bave a correct conception of 

 solids. It is really a lesson in solid geometry ; a subject which, as we 

 all know, many persons can only learn with great difficulty. But, 



