THE EYE AND THE PHYSIOLOGY OF VISION. 541 



In this manner there arises the complete image of the entire 

 object. Figure 167 shows this action of the lens for two 

 given points. Reference to it will also show that the image 

 will be inverted. This is also true of the image on the retina. 

 With these physical conceptions of the nature of light 

 and its properties we are ready to understand with more 

 meaning the detailed anatomy of the eye. 



Fig. 167. DIAGRAM TO ILLUSTRATE THE REFLECTION AND THE REFRACTION OF LIGHT. 

 At A the beam of light is reflected, illuminating the arrow a c, at B. From all points 

 of the arrow a c, the light is reflected in all directions. Two such divergent beams are 

 shown as refracted by the convex lens L L, and brought to points a' and c', producing at 

 these points images of the points a, c. As this is true for all points on a c, there will be a 

 complete image at a' c'. The lines at S' S', and S" S", show positions of screen where 

 the image is blurred. 



THE ANATOMY OF THE EYE. 



While the eyeball itself contains the sensitive retina and 

 the refracting media, and so the essential parts of the eye, 

 it is provided with a number of appendages which make its 

 use in vision much more efficient. On account of its sen- 

 sitiveness it needs special organs for protection. A system 

 of muscles enable it to make rapid movements, while a 

 lachrymal apparatus protects it from an accumulation of 

 dust and other foreign particles. 



1. The Eyebrows. The function of these is so ap- 

 parent from every-day experience, preventing the perspira- 

 tion of the forehead from running over the eyeball but 

 turning it aside down the cheeks, that further statement is 

 unnecessary. That they act in shading the eye is probably 

 only exceptionally true. 



