THE SPECIAL SENSES 



129 



near point; i.e., the nearest point to the eye at which 

 that object can be distinctly seen. Nearer than this the 

 object is blurred. If the convexity of the lens be in- 

 creased, if it becomes more nearly a perfect sphere, the 

 bending of the rays will be increased equally and the 

 focal point will be closer to the back of the lens. Con- 

 versely, flattening the lens would diminish the refrac- 

 tion and the focus would be at a more distant point be- 

 hind the lens. In order to see clearly, therefore, the eye 

 must have the power of accommodating itself to the dis- 

 tance of the objects, since the power of seeing at a fixed 



Fig. 30. Formation of image on retina. O.A. is the optic axis. (Pearce- 

 Macleod, Fundamentals of Human Physiology.) 



distance only would be of very little use. This purpose 

 might be accomplished either by moving the lens back- 

 wards and forwards or by flattening or bulging of the 

 lens. In some fishes the former method is observed, but 

 in mammals the latter is the uniform rule. This action 

 of the eye is called accommodation and is carried out by 

 a muscular arrangement to be explained. 



Inverted Imag-es, When an image is thrown through 

 a small opening, on a deeply concave mirror, the rays 

 from the top of the object reflected strike on the lower 

 and those from the bottom on the upper part of the 

 concavity. Hence the image is inverted. All objects 



