THE EYE 36l 



Why We See Objects Upright. You have noticed from all 

 experiments that the image made by a convex lens is always inverted ; 

 since the crystalline lens of the eye is a convex lens, the image formed 

 on the retina will be reversed. The sensation produced by the image 

 on the retina is conveyed into the brain by the optic nerve. It is 

 not the image but the object from which the rays of light come, 

 forming the image, on which the eye depends for an impression; and 

 since the eye deals with the outside object, and depends upon the 

 inside image for the sensation, the impression made on the brain 

 is that of the real object which is upright. If, however, we look 

 at the image formed by a convex lens the image is inverted. Again, 

 the eye is dealing with the object outside of the eye, which, in this 

 case, is an inverted image formed by a convex lens. The sensation 

 given to the brain is an inverted image which is the actual object 

 outside the eye. 



Optical Illusions. The sense of sight is liable to errors and illu- 

 sions. A few interesting examples will show how a person may be 

 deceived through the sense of sight. In the illustration the white 

 square will appear larger than the black one. Fig. 249. 



A vertical line which is thin will seem longer than a thick hori- 

 zontal line of the same length. 



People dressed in white look larger than when dressed in black. 

 Another interesting illusion is taken advantage of in decorating 

 rooms and in the selection of dresses. 



If two perfect squares be made, one of horizontal lines and the 

 other of vertical lines, the space covered by one set seems to be greater 

 than the space covered by the other set of lines. 



Stout people should not wear dress goods having horizontal 

 stripes, and slim people should avoid dress goods with vertical 

 stripes. 



Other optical illusions may be produced by crossing two parallel 

 lines with a series of oblique lines as shown in the illustrations. 



An interesting optical illusion may be produced by looking through a paper 

 tube with one eye, and holding a hand near the tube. If the other eye is open, 

 objects appear visible through an apparent hole in the hand. 



The poet was perfectly right when he said that things are not 



