348 



ADVANCED PHYSIOLOGY 



hand it is replaced by one of less curvature, it will bend them 

 less sharply and bring them to a focus at a greater distance 

 from the lens. If, however, the lens be concave, the rays 

 will be scattered instead of being brought to a focus. It is 

 important that these points be clearly seen, in order that one 

 may understand the accommodation of the eye. 



If, instead of a single point giving out light, there is an 

 object of some size, the action of the lens is essentially the 



same. Suppose that 

 the object is a candle 

 flame (Fig. 171); the 

 tip of the light, A, is 

 a point and the rays 

 passing from it will, 



of course, be focused 

 FIG. 171. DIAGRAM 



at 



The 



Showing the methodof the formation of an image 



a po i ntj 

 Q ^ 



another point, B, 

 which will be focused at b. In the center of the candle we 

 might select another point which would then be focused 

 between a and b. The whole candle and its flame are 

 made up of points, each of which will be focused at a cor- 

 responding point. If, therefore, a piece of paper or a screen 

 be placed at the line, a-b, we shall find an image of the candle 

 flame upon it. The image, however, will be upside down, as 

 is evident from the figure. 



In the human eye an image is formed in exactly the same 

 manner. The lens focuses the light that passes through it 

 and produces an image at a certain distance behind it. In 

 a normal eye the retina is at just the proper distance so that 

 the rays of light are focused upon it, and the image thus 

 formed; Fig. 172. Since this image is, of course, up side 

 down, one naturally asks, why do we not see things in in- 

 verted positions. This question arises from the false im- 

 pression that one's brain pictures things just as they occur on 



