ioo8 THE SENSES 



SECTION I. VISION. 



Physical Introduction. Physically, a ray of light is a series of dis- 

 turbances or vibrations in the luminiferous ether, which radiates out 

 from a luminous body in what is practically a straight line. The ether 

 is supposed to fill all space, including the interstices between the mole- 

 cules of matter and the atoms of which those molecules are composed. 

 Suppose a bar of iron to be gradually heated in a dark room. In the 

 cold iron the molecules are moving on the average at a relatively slow 

 rate, and the waves set up in the ether by their vibrations are compara- 

 tively long. Now, the. long ethereal vibrations do not excite the retina, 

 because it is only fitted to respond to the impact of the shorter waves ; 

 and, indeed, the long waves are largely absorbed by the watery media 

 of the eye. As the temperature of the iron bar is increased, the mole- 

 cules begin to move more quickly, and waves of smaller and smaller 

 length, of greater and greater frequency, are set up, until at last some 

 of them are just able to stimulate the retina, and the iron begins to glow 

 a dull red. As the heating goes on the molecules move more quickly 

 still, and, in addition to waves which cause the sensation of red, shorter 

 waves that give the sensation of yellow appear. Finally, when a high 

 temperature has been reached, the very shortest vibrations which can 



affect the eye at all mingle with the medium 

 and long waves, and the sensation is one of 

 intense white light. 



We have said that a ray of light travels 

 in a straight line, and the direction of the 

 straight line does not change as long as the 

 medium is homogeneous. But when a ray 

 reaches the boundary of the medium through 

 which it is passing, a part of it is in general 

 turned back or reflected. If the second 

 medium is transparent (water or glass, e.g.), 

 the greater part of the ray passes on through 

 it, a smaller portion is reflected. If the 

 Fig. 404. Reflection from a second medium is opaque, the ray does not 

 Plane Mirror. penetrate it for any great distance; if it is 



a piece of polished metal, e.g., nearly the 



whole of the light is reflected ; if it is a layer of lampblack, very little 

 of the light is reflected, most of it is absorbed. 



Reflection. The first law of reflection is that the reflected ray, the ray 

 which falls upon the reflecting surface (incident ray), and the normal to the 

 surface, are in one plane. The second law is that the reflected ray makes 

 with the perpendicular (normal) to the reflecting surface the same angle 

 as the incident ray. A corollary to this is that a ray perpendicular to 

 the surface is reflected along its own path. 



Reflection from a Plane Mirror. Let a ray of light coming from the 

 point P (Fig. 404) meet the surface DE at B, making an angle PBA with 

 the normal to the surface. The reflected ray BC will make an equal 

 angle ABC with the normal; and the eye at C will see the image of P 

 as if it were placed at P', the point where the prolongation of BC cuts 

 the straight line drawn from P perpendicular to DE. This is the posi- 

 tion of an ordinary looking-glass image. 



Reflection from a Concave Spherical Mirror. A spherical surface may 

 be supposed to be made up of an infinite number of infinitely small plane 

 surfaces. The normal to each of these plane surfaces is the radius of 

 the sphere, and the reflected ray makes with the radius at the point of 



