CHAPTER XIII 



THE SPECIAL SENSES 



Sight. The eye is a special organ by means of which certain 

 rhythmic disturbances of the ether affect consciousness and 

 produce the sensation of light. Among other functions that 

 the eye serves are the determination of color, of distance, and 

 of form. It consists of various adjustable refracting media, by 

 means of which the rays of light are focussed properly on the 

 retina, and of various muscles and accessory structures by 

 means of which the eye is moved in different directions and 

 is protected. 



The muscles of the eye serve to move the eyeball through wide 

 angles in all directions. When the axis of vision points straight 

 ahead, the eye is in the primary position. If it moves from this 

 position, so that the axis of vision rotates either about the trans- 

 verse or vertical axis, then the eye is in a secondary position. 

 All other positions are called tertiary. In order to understand 

 clearly the nature of the image received by the eye, it is only 

 necessary to review the images cast by a convex lens. If a 

 double convex lens is taken and the image formed by a luminous 

 object is noted, it is seen that it is an inverted image. Referring 

 to Fig. 14, it will be seen that the rays originating at A will 

 be twice refracted by the lens, once as they enter it and again 

 as they leave it, so that all rays from A reaching the lens are 

 joined at a. The same is true for B and 6. Therefore, a screen 

 placed at focus, F, will receive an inverted image, a&, of the 

 luminous object AB. If the lens were more convex, the image 

 would be formed nearer the lens; if the lens were flatter, the 

 image would fall farther from the lens. Again, on the other 

 hand, if the image is to be formed at a definite spot, the farther 

 the object is from the lens the flatter the lens must be; and, vice 

 versa, the nearer the object the more curved the lens must be, 



