2o6 THE POPULAR SCIENCE MONTHLY. 



and the spot can be detected only by the most careful inves- 

 tigation. 



Not the least of the wonders of the eye are connected with the 

 appreciation of images made upon the retina by certain parts of 

 the brain. It is literally true that a person may see and yet not 

 perceive. It has happened, in certain injuries of the brain, that 

 a person sees and reads the words in a book and yet does not per- 

 ceive their significance. This is called word-blindness. In a cer- 

 tain portion of the brain is a part which enables us to recognize the 

 fact that we see an object ; yet this object conveys no idea. There 

 are two of these so-called centers of vision, one on either side, 

 and their action is partly crossed. When the center is destroyed 

 on one side, the inner half of one eye and the outer half of the 

 other eye are blinded. Farther back in the brain, however, is a 

 center which enables us to perceive or understand what is seen. 

 "When this center is destroyed we see objects and may avoid ob- 

 stacles in walking, but persons, words, etc., are not recognized. 

 This center exists only on the left side of the brain. 



An impression, however short, made upon the retina is per- 

 ceived. The letters on a printed page are distinctly seen when 

 illuminated by an electric spark, the duration of which is only 

 forty billionths of a second ; but the impression remains much 

 longer. Anything in motion appears to us in a way quite differ- 

 ent from the single impression that we should have from an elec- 

 tric spark. In a picture representing an animal in motion, as it 

 appears in an instantaneous photograph, the positions seems ab- 

 surd and like nothing we have ever seen. In looking at a horse 

 in action, the impressions made by the different position of the 

 animal run into each other, and art should represent as nearly as 

 possible the sum or average of these impressions. It is also true 

 that impressions are diffused in the retina beyond the points upon 

 which they are directly received. This is called irradiation ; and 

 the impression is diffused farther for white or light-colored than 

 for black or dark objects. It is well known that a white square 

 looks considerably larger than a dark square of exactly the same 

 size ; or the hands in white gloves look larger than in black gloves. 



I have described, in as simple a way as possible, some won- 

 derful things about the eye ascertained and explained by mod- 

 ern investigations; but there are many interesting facts ascer- 

 tained which space has not permitted me to discuss, and there 

 still remains much that is not yet understood. The whole ques- 

 tion of the appreciation of colors and of color-blindness is still 

 wrapped in mystery. We know that some persons can not 

 distinguish between certain colors, but the reason of this is ob- 

 scure. Perfect sight can exist only when the eye is perfect. The 

 form and color of objects may be distorted so that an inaccurate 



