o8 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Higher and Lower Senses.— The lower senses 

 are those in which contact is direct — viz., feeling, taste, 

 and smell. The higher senses are those in which the im- 

 pression is through the vibrations of a medium, viz., 

 hearing and sight. When the impression is indirect — i. e. 

 through a medium— besides the specialized nerve there 

 is also a mechanical instrument — acoustic in the case of 

 the ear and optic in the case of the eye— placed in front 

 of the specialized nerve in order to make the impression 

 stronger and more definite. The specialized nerve, to- 

 gether with the mechanical instrument, constitute the 

 sense organ. On these two higher senses, therefore, are 

 founded all our fine art and nearly all our science. 



SENSE OF SIGHT AND ITS ORGAN, THE EYE. 



The most specialized of all sensory fibers are those of 

 the optic nerve ; the most refined of all instruments is the 

 eye; the highest of all the senses is the sense of sight. 

 And yet we are apt to greatly overestimate what is 

 really given by this sense. The direct gifts of sight are 

 light, its intensity, its color, and its direction, nothing more. 

 All else, as size, distance, relief-form, etc., are judgments. 

 This will be explained hereafter. 



Again, we must distinguish between light objective and 

 light subjective. Thcone consists of vibrations of the ether 

 and exist independent of us ; the other is the peculiar sen- 

 sation produced in us or in some other percipient by these 

 vibrations. The one belongs to physics, the other to 

 physiology, or perhaps, some may claim, to psychology. 

 We are here concerned mainly with light as sensation ; 

 but since this is produced by vibrations, we shall be com- 

 pelled to say something of the physics of the subject also. 



Primary Divisions of the Subject. — The phenom- 

 ena of vision may be divided into two primary groups: 

 monocular and binocular. Monocular vision includes 



