THE EYE AS AN OPTICAL INSTRUMENT 219 



The Ciliary Muscle. (7, Fig. 81.) Between the sclerotic and 

 choroid coats, just where the former merges into the cornea, are 

 small masses of smooth muscle-fibers which make up the ciliary 

 muscle. These fibers are attached in front to the sclerotic coat and 

 pass back a short distance to an insertion in the choroid coat just 

 in front of the ciliary processes. The contraction of the ciliary 

 muscle pulls the margin of the choroid coat forward and inward. 

 The effect of this is to bring the ciliary processes nearer together 

 and loosen the suspensory ligament, which is attached to them. 

 The tension upon the capsule of the crystalline lens is thus di- 

 minished. 



The ciliary muscle is interesting as being the only voluntary 

 muscle in the Body which is innervated through the sympathetic 

 system. 



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. Starting 

 from a luminous point light travels in all directions along the 

 radii of a sphere of which the point is the center; the light propa- 

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

 ethereal particles vibrate from side to side in a plane perpendic- 

 ular to the direction of the ray. 



Any ray, all of whose particles are vibrating at the same rate, 

 is a ray of monochromatic light. It has a pure spectral color. The 

 wave length of a beam of monochromatic light is measured by the 

 distance between any ethereal particle of the beam and the next 

 one which is in precisely the same phase of vibration. Since the 

 rate at which light travels is nearly fixed, the wave length must 

 vary inversely as the vibration rate. Light of high vibration rate 

 has short wave length and vice versa. The color of monochro- 

 matic light depends upon its wave length. Where lights of va- 

 rious wave lengths are mixed together in a beam a compound light 

 results. To the eye such a beam gives a definite color sensation 

 but not one of the pure spectral colors. 



Refraction. When light passes obliquely from one transpar- 

 ent medium into another of different density it is bent from its 



