892 A MANUAL OF PHYSIOLOGY 



VISION. 



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

 disturbances 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 molecules 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 comparatively long. Now, the long ethereal vibra- 

 tions 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 tempera- 

 ture of the iron bar is increased, the molecules 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 sen- 

 sation 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 homo- 

 geneous. But when a ray reaches the 

 FIG. 369. REFLECTION FROM boundary of the medium through which 

 A PLANE MIRROR. 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 second 

 medium is opaque, the ray does not penetrate it for any great dis- 

 tance ; 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. 369) 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 pro- 

 longaton of BC cuts the straight line drawn from P perpendicular 

 to DE. This is the position of an ordinary looking-glass image. 



