Page:Popular Science Monthly Volume 28.djvu/600

582 used, by far the greater part of any class will, from mere force of habit, commit the contents, and then imagine they see everything mentioned in the books and nothing more. After they have been trained to observe, they may be allowed to consult books, but not before. what is true of books is true of lectures on objects taught in the laboratory. The students always wish to have the lecture first and see the object afterward. It seems to them to lighten the work, but they fail to recognize, what is evident to the instructor, that they are not learning so much or so well.

Again, few students 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 purpose I use pieces of pine-wood which are given to the class early in the term, just as soon as they have 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 tangent, and in the order named. After they have made and drawn the first section, if asked what they think is the 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 like 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 they 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 the 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. Finally, 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 have 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,