AND PHYSIOLOGY. 



moved, nature will soon replace it ; but this is true of no 

 other medium. The aqueous humor, however, is of great use 

 to keep the front parts of the eye in a soft and elastic condi- 

 tion, and also to furnish a medium in which the iris may 

 readily be contracted and relaxed. It also aids in properly 

 refracting the rays of light, so that the most perfect image 

 may be formed on the retina. The use of the crystalline lens 

 is also to refract the rays of light, so that an image of suffi- 

 ciently small size may be formed on the retina. Otherwise, 

 when we look at any object larger than the diameter of the 

 pupil, we should be unable to see it, only by successively 

 looking at minute portions of it. The vitreous humor assists 

 in the same refraction of the rays, and incidentally subserves 

 the purpose of fixing at a proper distance the lens, so that the 

 image may be formed exactly at that point on the retina called 

 the focus, where it will be the most distinct. 



695. Need of Three Media.— But what is the necessity for 

 three media of refraction, and why will not the crystalline lens 

 answer the whole purpose ? This is owing to some of the 

 properties of light. Light is composed of seven rays or colors, 

 some of which are more easily refracted than others ; that is, 

 some rays are bent farther from a perpendicular to the sur- 

 face than others. Hence, upon the edges of an image formed 

 by a single lens, every part of which is of the same density, 

 we should see several of the prismatic colors, which would give 

 an indistinct image. In the telescope and microscope this 

 difficulty is remedied by forming a lens of crown and flint 

 glass, one of which has a stronger dispersive or refractive 

 power than the other. In the eye the same error is obviated 

 by means of the crystalline lens, which has a greater refrac- 

 tive power than the vitreous humor, and consequently, when 

 the ray is too much refracted by the crystalline lens, a corn- 



state the use of the aqueous. The use of the lens. What does the vitreous humor aid 

 in ? 695. What is the need of three humors ? How many distinct colors does light con- 

 sist of ? What similar purpose do lenses of different densities answer in telescopes? 

 Which of the three humors has the greatest refractive power ? 



