THE EYE AS AN OPTICAL INSTRUMENT. 517 



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 direction 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 travelling 

 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 oscillation 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 arc all 

 equal is called mono chromatic 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, 

 we come at definite distances along a ray, to particles in 

 the same phase of oscillation, say at their greatest distance 

 from their position of rest; just as in the concentric waves 

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

 any radius meet, at intervals, with water raised most above 

 its horizontal plane as the crest of a wave, or depressed most 

 below it as the hollow of a wave. The distance along the ray 

 from crest to crest is called a wave-length and is always the 

 same in any given simple light; but it is different in simple 



