Chap. 17 RESPONSIVENESS THE SENSE ORGANS 321 



pushing it inward. This starts corresponding vibrations that run through the 

 fluid for the length of the vestibular cavity and on into the tympanic cavity 

 toward the round window. As the vibrations travel along the cavities, each 

 one vibrates the basilar membrane and the sound receptors, more or less 

 strongly and in difterent regions, depending on its own character. Finally, 

 the vibration expends its force against the membrane of the round window 

 which it bends outward a little toward the middle ear. Our ability to dis- 

 tinguish different tones is due to the fact that the vibrations of a particular 

 tone pass more frequently through a certain part of the basilar membrane. 

 The nerve fibers ending in that part carry the impression of the tone to the 

 brain. 



Vision — Photoreceptors 



Light filters through the air in one direction; if it enters water it passes on 

 in a different direction. This change in direction is refraction and it occurs in 

 greater or lesser degree whenever light passes from one medium into another. 

 The amount of change in the direction depends upon the character of the new 

 medium and the angle at which the light enters it (Fig. 17.10). 



Lenses. A lens is a transparent object with a curved surface. A drop of 

 water is a lens. The lens in the eye of a frog or a man contains thousands of 

 cells. Artificial lenses are commonly made of glass, of quartz, or of fluorite. 

 When the surface of a biconvex lens is properly curved all the rays that enter 

 it are brought to a focal point at a certain distance from it, called the focal 

 distance. This distance varies with the curvature of the lenses in cameras 

 and microscopes, as well as with the curvature of the cornea and lens in 

 the human eye. 



There are various shapes of artificial lenses; the common foundation lens 



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Fig. 17.10. Formation of an image by a lens. Rays of light are reflected from 

 each point of a black arrow pointed end up. Rays from the right of the arrow are 

 intercepted by a glass lens and their courses are bent. Those from the lower end of 

 the arrow are turned upward; those from the upper end are turned downward. 

 Rays from every point of the black arrow are brought to a focus in a point beyond 

 the lens. These points compose a reversed image, shown by a white arrow pointed 

 end down. We see everything upside down. On the retina the legs of a horse point 

 up. The interpretation of the brain points them down. (Courtesy, Walls: The Ver- 

 tebrate Eye. Bloomfield Hills, Mich., The Cranbrook Press, 1942.) 



