Part 3

A good example to begin with is Fig. 8. These outlines will probably suggest at first view a book, or better a book cover, seen with its back toward you and its sides sloping away from you; but it may also be viewed as a book opened out toward you and presenting to you an inside view of its contents. Should the change not come readily, it may be facilitated by thinking persistently of the appearance of an open book in this position. The upper portion of Fig. 9 is practically the same as Fig. 8, and if the rest of the figure be covered up, it will change as did the book cover; when, however, the whole figure is viewed as an arrow, a new conception enters, and the apparently solid book cover becomes the _flat_ feathered part of the arrow. Look at the next figure (Fig. 10), which represents in outline a truncated pyramid with a square base. Is the smaller square nearer to you, and are the sides of the pyramid sloping away from you toward the larger square in the rear? Or are you looking into the hollow of a truncated pyramid with the smaller square in the background? Or is it now one and now the other, according as you decide to see it? Here (Fig. 13) is a skeleton box which you may conceive as made of wires outlining the sides. Now the front, or side nearest to me, seems directed downward and to the left; again, it has shifted its position and is no longer the front, and the side which appears to be the front seems directed upward and to the right. The presence of the diagonal line makes the change more striking: in one position it runs from the left-hand _rear_ upper corner to the right-hand _front_ lower corner; while in the other it connects the left-hand _front_ upper corner with the right-hand _rear_ lower corner.

Fig. 15 will probably seen at first glimpse to be the view of a flight of steps which one is about to ascend from right to left. Imagine it, however, to be a view of the under side of a series of steps; the view representing the structure of overhanging solid masonwork seen from underneath. At first it may be difficult to see it thus, because the view of steps which we are about to mount is a more natural and frequent experience than the other; but by staring at it with the intention of seeing it differently the transition will come, and often quite unexpectedly.

The blocks in Fig. 16 are subject to a marked fluctuation. Now the black surfaces represent the bottoms of the blocks, all pointing downward and to the left, and now the black surfaces have changed and have become the tops pointing upward and to the right. For some the changes come at will; for others they seem to come unexpectedly, but all are aided by anticipating mentally the nature of the transformation. The effect here is quite striking, the blocks seeming almost animated and moving through space. In Fig. 17 a similar arrangement serves to create an illusion as to the real number of blocks present. If viewed in one way--the black surface forming the tops of the blocks--there seem to be six arranged as in Fig. 18; but when the transformation has taken place and the black surfaces have become the overhanging bottoms of the boxes, there are seven, arranged as in Fig. 19. Somewhat different, but still belonging to the group of ambiguous figures, is the ingenious conceit of the duck-rabbit shown in Fig. 20. When it is a rabbit, the face looks to the right and a pair of ears are conspicuous behind; when it is a duck, the face looks to the left and the ears have been changed into the bill. Most observers find it difficult to hold either interpretation steadily, the fluctuations being frequent, and coming as a surprise.

All these diagrams serve to illustrate the principle that when the objective features are ambiguous we see one thing or another according to the impression that is in the mind's eye; what the objective factors lack in definiteness the subjective ones supply, while familiarity, prepossession, as well as other circumstances influence the result. These illustrations show conclusively that seeing is not wholly an objective matter depending upon what there is to be seen, but is very considerably a subjective matter depending upon the eye that sees. To the same observer a given arrangement of lines now appears as the representation of one object and now of another; and from the same objective experience, especially in instances that demand a somewhat complicated exercise of the senses, different observers derive very different impressions.

Not only when the sense-impressions are ambiguous or defective, but when they are vague--when the light is dim or the forms obscure--does the mind's eye eke out the imperfections of physical vision. The vague conformations of drapery and make-up that are identified and recognized in spiritualistic _séances_ illustrate extreme instances of this process. The whitewashed tree or post that momentarily startles us in a dark country lane takes on the guise that expectancy gives it. The mental predisposition here becomes the dominant factor, and the timid see as ghosts what their more sturdy companions recognize as whitewashed posts. Such experiences we ascribe to the action of suggestion and the imagination--the cloud "that's almost in shape like a camel," or "like a weasel," or "like a whale." But throughout our visual experiences there runs this double strain, now mainly outward and now mainly inward, from the simplest excitements of the retina up to the realms where fancy soars freed from the confines of sense, and the objective finds its occupation gone.

FOOTNOTE:

[5] In order to obtain the effects described in the various illustrations it is necessary in several cases to regard the figures for a considerable time and with close attention. The reader is requested not to give up in case the first attempt to secure the effect is not successful, but to continue the effort for a reasonable period. Individuals differ considerably in the readiness with which they obtain such effects; in some cases, such devices as holding the diagrams inverted or at an angle or viewing them with the eyes half closed are helpful.

NATURE STUDY IN THE PHILADELPHIA NORMAL SCHOOL.

BY L. L. W. WILSON, PH. D.

When it was first proposed to me to write for the Popular Science Monthly a brief account of the biological laboratories in the Philadelphia Normal School, and of the Nature work carried on under my direction in the School of Observation and Practice, I felt that I could not do justice either to the place or the work; for, in my judgment, the equipment of the laboratories and the work done in connection with them are finer than anything else of the kind either in this country or abroad--a statement which it seemed to me that I could not make with becoming modesty. But, after all, it is not great Babylon that I have built, but a Babylon builded for me, and to fail to express my sense of its worth is to fail to do justice to Dr. W. P. Wilson, formerly of the University of Pennsylvania, to whom their inception was due; to Mr. Simon Gratz, president of the Board of Education, who from the beginning appreciated their value, and without whose aid they never would have taken visible form; to the principals of the two schools, and, above all, to my five assistants, whose knowledge, zeal, and hard work have contributed more than anything else to the rapid building up of the work.

THE LABORATORIES AND THEIR EQUIPMENT.--The rooms occupied by the botanical and zoölogical departments of the normal school measure each seventy by twenty feet. A small workroom for the teachers cuts off about ten feet of this length from each room. In the middle of the remaining space stands a demonstration table furnished with hot and cold water. Each laboratory is lighted from the side by ten windows. From them extend the tables for the students. These give plenty of drawer space and closets for dissecting and compound microscopes. Those in the zoölogical room are also provided with sinks. Each student is furnished with the two microscopes, stage and eyepiece micrometers, a drawing camera, a set of dissecting instruments, glassware, note-books, text-books, and general literature.

The walls opposite the windows are in both rooms lined with cases, in which there is a fine synoptic series.

In the botanical laboratory this systematic collection begins with models of bacteria and ends with trees. In other cases, placed in the adjoining corridor, are representatives, either in alcohol or by means of models, of most of the orders of flowering plants, as well as a series illustrating the history of the theory of cross-fertilization, and the various devices by which it is accomplished; another, showing the different methods of distribution of seeds and fruits; another, of parasitic plants; and still another showing the various devices by means of which plants catch animals.

As an example of the graphic and thorough way in which these illustrations are worked out, the pines may be cited. There are fossils; fine specimens of pistillate and staminate flowers in alcohol; cones; a drawing of the pollen; large models of the flowers; models of the seeds, showing the embryo and the various stages of germination; cross and longitudinal sections of the wood; drawings showing its microscopic structure; pictures of adult trees; and samples illustrating their economic importance. For the last, the long-leaved pine of the South is used, and samples are exhibited of the turpentine, crude and refined; tar and the oil of tar; resin; the leaves; the same boiled in potash; the same hatcheled into wool; yarn, bagging and rope made from the wool; and its timber split, sawn, and dressed.

The series illustrating the fertilization of flowers begins with a large drawing, adapted by one of the students from Gibson, showing the gradual evolution of the belief in cross-fertilization from 1682, when Nehemiah Grew first declared that seed would not set unless pollen reached the stigma, down to Darwin, who first demonstrated the advantages of cross-fertilization and showed many of the devices of plants by which this is accomplished. The special devices are then illustrated with models and large drawings. First comes the dimorphic primrose; then follows trimorphic _Lythrum_, to the beautiful model of which is appended a copy of the letter in which Darwin wrote to Gray of his discovery:

"But I am almost stark, staring mad over Lythrum.... I should rather like seed of Mitchella. But, oh, Lythrum!

"Your utterly mad friend, "C. DARWIN."

Models of the cucumber, showing the process of its formation, and the unisexual flowers complete this series. Supplementing this are models and drawings of a large number of flowers, illustrating special devices by which cross-fertilization is secured, such as the larkspur, butter and eggs, orchids, iris, salvia, several composites, the milkweed, and, most interesting of all, the Dutchman's pipe. This is a flower that entices flies into its curved trumpet and keeps them there until they become covered with the ripe pollen. Then the hairs wither, the tube changes its position, the fly is permitted to leave, carrying the pollen thus acquired to another flower with the same result.

Pictures and small busts of many naturalists adorn both of the rooms. Of these the most notable is an artist proof of Mercier's beautiful etching of Darwin. Every available inch of wall space is thus occupied, or else, in the botanical laboratory, has on it mounted fungi, lichens, seaweeds, leaf cards, pictures of trees, grasses, and other botanical objects.

The windows are beautiful with hanging plants from side brackets meeting the wealth of green on the sill. Here are found in one window ferns, in another the century plant; in others still, specimens of economic plants--cinnamon, olive, banana, camphor. On the tables are magnificent specimens of palms, cycads, dracænas, and aspidistras, and numerous aquaria filled with various water plants. Most of these plants are four years old, and all of them are much handsomer than when they first became the property of the laboratory. How much intelligent and patient care this means only those who have attempted to raise plants in city houses can know.

The zoölogical laboratory is quite as beautiful as the botanical, for it, too, has its plants and pictures. It is perhaps more interesting because of its living elements. Think of a schoolroom in which are represented alive types of animals as various as these: amoeba, vorticella, hydra, worms, muscles, snails and slugs of various kinds, crayfish, various insects, including a hive of Italian bees, goldfish, minnows, dace, catfish, sunfish, eels, tadpoles, frogs, newts, salamanders, snakes, alligators, turtles, pigeons, canaries, mice, guinea-pigs, rabbits, squirrels, and a monkey! Imagine these living animals supplemented by models of their related antediluvian forms, or fossils, by carefully labeled dissections, by preparations and pictures illustrating their development and mode of life; imagine in addition to this books, pamphlets, magazines, and teachers further to put you in touch with this wonderful world about us, and you will then have some idea of the environment in which it is the great privilege of our students to live for five hours each week.

In addition to these laboratories there is a lecture room furnished with an electric lantern. Here each week is given a lecture on general topics, such as evolution and its problems, connected with the work of the laboratories.

THE COURSE OF STUDY PURSUED BY THE NORMAL STUDENTS.--Botany: In general, the plants and the phenomena of the changing seasons are studied as they occur in Nature. In the fall there are lessons on the composites and other autumn flowers, on fruits, on the ferns, mosses, fungi, and other cryptogams. In the winter months the students grow various seeds at home, carefully drawing and studying every stage in their development. Meanwhile, in the laboratory, they examine microscopically and macroscopically the seeds themselves and the various food supplies stored within. By experimentation they get general ideas of plant physiology, beginning with the absorption of water by seeds, the change of the food supply to soluble sugar, the method of growth, the functions, the histology, and the modifications of stem, root, and leaves. In the spring they study the buds and trees, particularly the conifers, and the different orders of flowering plants.

The particular merit of the work is that it is so planned that each laboratory lesson compels the students to reason. Having once thus obtained their information, they are required to drill themselves out of school hours until the facts become an integral part of their knowledge.

For the study of fruits, for example, they are given large trays, each divided into sixteen compartments, plainly labeled with the name of the seed or fruit within. Then, by means of questions, the students are made to read for themselves the story which each fruit has to tell, to compare it with the others, and to deduce from this comparison certain general laws.

After sufficient laboratory practice of this kind they are required to read parts of Lubbock's Flower, Fruit, and Leaves, Kerner's Natural History of Plants, Wallace's Tropical Nature, and Darwinism, etc.

Finally, they are each given a type-written summary of the work, and after a week's notice are required to pass a written examination.

Zoölogy: The course begins in the fall with a rather thorough study of the insects, partly because they are then so abundant, and partly because a knowledge of them is particularly useful to the grade teacher in the elementary schools.

The locust is studied in detail. Tumblers and aquaria are utilized as vivaria, so that there is abundant opportunity for the individual study of living specimens. Freshly killed material is used for dissection, so that students have no difficulty in making out the internal anatomy, which is further elucidated with large, home-made charts, each of which shows a single system, and serves for a text to teach them the functions of the various organs as worked out by modern physiologists.

They then study, always with abundant material, the other insects belonging to the same group. They are given two such insects, a bug, and two beetles, and required to classify them, giving reasons for so doing. While this work is going on they have visited the beehive in small groups, sometimes seeing the queen and the drone, and always having the opportunity to see the workers pursuing their various occupations, and the eggs, larvæ, and pupæ in their different states of development. Beautiful models of the bees and of the comb, together with dry and alcoholic material, illustrate further this metamorphosis, by contrast making clearer the exactly opposite metamorphosis of the locust.

At least one member of each of the other orders of insects is compared with these two type forms, and, although only important points are considered at all, yet from one to two hours of laboratory work are devoted to each specimen. This leisurely method of work is pursued to give the students the opportunity, at least, to think for themselves. When the subject is finished they are then given a searching test. This is never directly on their required reading, but planned to show to them and to their teachers whether they have really assimilated what they have seen and studied.

After this the myriapods, the earthworm, and peripatus are studied, because of their resemblance to the probable ancestors of insects. In the meantime they have had a dozen or more fully illustrated lectures on evolution, so that at the close of this series of lessons they are expected to have gained a knowledge of the methods of studying insects, whether living or otherwise, a working hypothesis for the interpretation of facts so obtained, and a knowledge of one order, which will serve admirably as a basis for comparison in much of their future work.

They then take up, more briefly, the relatives of the insects, the spiders and crustaceans, following these with the higher invertebrates, reaching the fish in April. This, for obvious reasons, is their last dissection. But with living material, and the beautiful preparations and stuffed specimens with which the laboratory is filled, they get a very general idea of the reptiles, birds, and mammals. This work is of necessity largely done by the students out of school hours. For example, on a stand on one of the tables are placed the various birds in season, with accompanying nests containing the proper quota of eggs. Books and pamphlets relating to the subject are placed near. Each student is given a syllabus which will enable her to study these birds intelligently indoors and out, if she wishes to do so.

In the spring are taken up the orders of animals below the insect, and for the last lesson a general survey of all the types studied gives them the relationships of each to the other.

THE COURSE OF STUDY PURSUED IN THE SCHOOL OF PRACTICE.--In addition to the plants and animals about them, the children study the weather, keeping a daily record of their observations, and summarizing their results at the end of the month. In connection with the weather and plants they study somewhat carefully the soil and, in this connection, the common rocks and minerals of Philadelphia--gneiss, mica schist, granite, sandstone, limestones, quartz, mica, and feldspar.

As in the laboratories, so here the effort is made to teach the children to reason, to read the story told by the individual plant, or animal, or stone, or wind, or cloud. A special effort is made to teach them to interpret everyday Nature as it lies around them. For this reason frequent short excursions into the city streets are made. Those who smile and think that there is not much of Nature to be found in a city street are those who have never looked for it. Enough material for study has been gathered in these excursions to make them a feature of this work, even more than the longer ones which they take twice a year into the country.