March 7, 1913] 



SCIENCE 



359 



ing with which they should leave them, at 

 the same time gradually increasing their 

 fund of concrete, definite, dependable and 

 useful information. 



This method makes it possible to master 

 fewer principles in a given time; but, as 

 the psychologists have conclusively proved, 

 assures the pupils of a much greater chance 

 of retaining both the subject matter studied 

 and the methods of reasoning used as real 

 helps in solving the real problems of later 

 life. In other words, the method de- 

 manded by the practical philosophy is the 

 one that assures us of giving the greatest 

 amount of transferable training.^ 



In order to fix in mind the differences 

 among the three types of method just de- 

 scribed, the following three samples of 

 treatment are given. They are typical of 

 the way in which the subject of light may 

 be introduced in accordance with the three 

 types of physical philosophy. 



I. Encyclopedic. — A luminous body is 

 one that emits light. A medium is any 

 substance through which light passes. A 

 transparent body is one that obstructs light 

 so little that we can see objects through it. 

 A translucent body is one that lets some 

 light pass, but not enough to render objects 

 visible through it. An opaque body is one 

 that does not transmit light. A ray of 

 light is a single line of light. A pencil or 

 beam of light is a collection of rays, which 

 may be parallel, diverging or converging; 

 it may be traced in a dark room into which 

 a sunbeam is admitted by the floating par- 

 ticles of dust which reflect the light to the 

 eye. 



The visual angle is the angle formed at 

 the eye by rays coming from the extremi- 

 ties of an object. Knowing the distance 



' For a more detailed discussion of this point, 

 see Mann, ' ' The Teaching of Physics for Purposes 

 of General Education," Chap. VII.-X. New 

 York, Macmillan, 1912. 



of a body, we immediately estimate its size 

 by the visual angle. 



Laws of Light. (1) Light passes off 

 from a luminous body equally in all direc- 

 tions. (2) Light travels through a uni- 

 form medium in straight lines. (3) The 

 intensity of light decreases as the square 

 of the distance increases. 



II. Abstract. — Just as sound is defined 

 as undulations in the air, or some other 

 medium, that produce the sensation we call 

 sound, so light, in the same sense, consists 

 of imdulations or waves in a medium that 

 produce the sensation called light. Physi- 

 cists have agreed to call this medium which 

 transmits light the ether. It exists every- 

 where, even penetrating between the mole- 

 cules and atoms of ordinary matter. Noth- 

 ing is known about its nature and but little 

 concerning the exact way in which light 

 travels through it; but the masters of sci- 

 ence generally agree that light is a wave 

 motion in the ether, and that the vibrations 

 of these waves are not longitudinal as in 

 sound waves, but transverse. The trans- 

 verse disturbances by means of which the 

 waves are propagated are probably not 

 transverse physical movements of the ether, 

 but transverse alterations in its electrical 

 and magnetic conditions. 



A transparent body is one which allows 

 light to pass through it with so little loss 

 that objects can easily be distinguished 

 through it. Examples of transparent 

 bodies are glass, air, water. A body is 

 translucent when it transmits light so im- 

 perfectly that objects can not be seen dis- 

 tinctly through it. Such bodies are horn, 

 oiled paper, thin sheets of wood. Opaque 

 bodies are those which transmit no light, as 

 brick, pig iron, wooden boards. No sharp 

 line of separation between these classes 

 can be drawn; the classification is one of 



III. Concrete. — If a number of .people 



