360 



SCIENCE 



[N. S. Vol. XXXVII. No. 949 



are asked how large the moon looks, each 

 will give a different answer. One may say 

 that it looks as large as a dime, another 

 that it seems as large as a saucer, while a 

 third may say that it looks as large as a 

 cart wheel. Then, too, the moon looks 

 larger to every one when it is near the 

 horizon than when it is high in the sky. 



Infants reach for the moon and cry be- 

 cause they can not get it. Landsmen find 

 it very difficult to estimate the distance 

 between two boats at sea. On the other 

 hand, when we look at a man climbing a 

 distant hill, he appears as but a small speck 

 on the landscape, yet we estimate his size 

 correctly. We even use our knowledge of 

 the man's size to estimate the distance or 

 actual size of the hill or the height of the 

 trees there. Ability to estimate distances 

 and sizes from the way things look is ob- 

 tained from long practise. Let us see if 

 we can find the reasons for these things. 



When sunlight streams through the win- 

 dow, it traces an outline of the window on 

 the floor. If you hold your open hand so 

 that the sunlight falls vertically upon it, 

 the outline of the shadow cast on the floor 

 resembles the outline of the hand. Most 

 of us have amused ourselves making 

 shadow pictures, by so placing the hands 

 between a lamp and the wall that the 

 shadow on the wall resembled a rabbit, a 

 goose, a clown, or any other creature. We 

 might draw the same outline by pivoting 

 one end of a long straight pencil at the 

 source of light, and moving it around the 

 edges of the object, while the other end 

 marked on a paper suitably placed. We 

 can think of such a pencil as if it were the 

 beam from a tiny searchlight moving about 

 the edges of the object and tracing the 

 outline. 



When a sunbeam is allowed to enter a 

 darkened room through a small opening, 

 its path, as revealed by the dust particles 

 in the air, is seen to be a straight line. 



Where it falls on some object it makes a 

 bright spot. The sun, the opening, and the 

 bright spot all lie on the same straight 

 line; so from inside the darkened room we 

 can determine the direction of the sun with 

 reference to objects in the room, by means 

 of the line drawn from the center of the 

 bright spot through the center of the open- 

 ing. Because light travels in straight 

 lines, we judge the direction of an object 

 by observing the direction in which light 

 from the object travels. 



Whatever you may think of the relative 

 merits of the three types of method just 

 oiitlined, it is clear that the only way to 

 bring physics close to daily life is to bring 

 daily life close to physics. The only 

 method that assures the teacher of doing 

 this successfully is that of the practical or 

 concrete philosophy. It is possible that 

 other methods may be more successful 

 when the aim is to prepare students to 

 meet past or present college entrance re- 

 quirements, or to pursue later courses in 

 som'e of the technical schools. Other 

 methods can not, however, compete with 

 the concrete method when the aim of the 

 teaching is the union of education and life. 

 Each teacher must, therefore, choose his 

 own aim and adapt his methods to suit it. 

 Let me in closing remind you of the im- 

 portance of the choice. Had education 

 and life been united long ago, the schools 

 would not now stand discredited, nor would 

 the demand for separate vocational schools 

 have arisen. A union now of education 

 and life will save the situation. 



C. R. Mann 



The University of Chicago 



ON THE APPEABANCE OF BELIVM AND 

 NEON IN VACUUM TUBES ^ 



At the last meeting of the Chemical 

 Society, Sir William Ramsay, Prof. Collie, 

 ' From Nature. 



