492 THE HUMAN BODY. 



The Properties of Light. Before proceeding to the 

 study of the eye as an optical instrument, it is necessary to 

 recall briefly certain properties of light. 



Light is considered as a form of movement of the particles 

 of an hypothetical medium, or ether, the vibrations being in 

 planes at right angles to the line of propagation of the 

 light. "When a stone is thrown into a pond a series of 

 circular waves travel from that point in a horizontal direc- 

 tion over the water, while the particles of water themselves 

 move up and down, and cause the surface inequalities 

 which we see as the waves. Somewhat similarly, light-waves 

 spread out from a luminous point, but in the same medium 

 travel equally in all directions so that the point is surrounded 

 by shells of spherical waves, instead of rings of circular 

 waves traveling in one plane only, as those on the surface 

 of the water. Starting from a luminous point light 

 would travel in all directions along the radii of a sphere of 

 which the point is the centre; the light propagated along 

 one such radius is called a ray, and in each ray the ethereal 

 particles swing from side to side in a plane perpendicular 

 to the direction of the ray. Taking a particle on any ray 

 it would swing aside a certain distance from it, then back 

 to it again, and across for a certain distance on the other 

 side; and then back to its original position on the line of 

 the ray. Such a movement is an oscillation, and takes a 

 certain time; in lights of certain kinds the periods of oscilla- 

 tion are all the same, no matter how great the extent or 

 amplitude of the oscillation; just as a given pendulum will 

 always complete its swing in the same time no matter 

 whether its swings be great or small. Light composed of 

 rays in which the periods of oscillation are all equal is 

 called monochromatic or simple light, while light made of a 

 mixture of oscillations of different periods is called mixed 

 or compound light. 



If monochromatic light is steadily emitted from a point, 

 then, at certain distances along a ray, we come to particles 

 in the same phase of oscillation, say at their greatest dis- 

 tance from their position of rest; just as in' the concentric 

 waves seen on the water after throwing in a stone we would 



