170 



THE SENSES. 

 FIG. 50. 



Formation of Image by Convex Lens. 



Does the crystalline lens throw an inverted image upon the 



retina ? 



Yes. This may easily be demonstrated by looking at the image 

 from behind a fresh eye of an albino animal (white rabbit) or if 

 the sclera be thinned. 



How is this inversion corrected? 



The correction is made by the brain in its perception of the image. 

 It is an act of mental and not of physical origin. Thus, objects 

 which are projected upon the left of the retinal surface look to be, 

 as they are, on the right of the body ; and so with all the direc- 

 tions : the inversion of the retinal image is corrected by the mind. 



What other visual perceptions are the result of mental processes ? 

 We are able to estimate by the aid of the brain the size, direc- 

 tion, distance, form, and speed of motion of a thing which we fyave 

 seen. All of these are judgments based largely upon previous 

 experience. All of these deductions are liable to error by reason 

 of faulty judgment or faulty vision, but this is the usual method 

 of forming such estimates. 



How does the eye perceive colors ? 



It is probable that particular rods and cones are capable of re- 

 sponding to rays of light of a certain wave-length, and to those rays 

 alone. It is well known that the rays of red light are of a certain 

 length of vibration. The same is true of yellow and of green rays. 

 We can conceive that each primary color has its own set of cones 

 and rods capable of responding to its stimulus, and that by combi- 

 nations of these stimuli the complementary colors and variations of 

 shade may be perceived by the resulting stimuli acting upon the 

 brain-centres. Such teaching is, of course, speculative, but this is 

 one theory which has acceptance. 



