PROTECTION BY COLOR. 163 



we do not know of its existence, but it is there. If the bird's 

 breast were the same color as his back, the shadow, mak- 

 ing it appear darker than it is (that is, darker than the back), 

 would bring out the line of the breast sharply against the 

 background. The shadow on a light breast cancels the effect 

 of light upon a dark back and causes the outline to blend with 

 the background. 



Nothing could be simpler than the experiment by which 

 Mr. Abbott H. Thayer, the artist who painted the ^' Madonna 

 Enthroned " and other well-known pictures, proved his discovery 

 of this " law of gradation " to a large number of scientists. Any 

 child can perform the experiment with very little trouble. We 

 quote from the original report of the experiment : " Mr. Thayer 

 placed three sweet potatoes, or objects of corresponding shape 

 and size, horizontally on a wire a few inches above the ground. 

 They were covered with some sticky material, and dry earth 

 from the road on which they stood was sprinkled over them so 

 that they would be of the same color as the background. The 

 two end ones were then painted white on the under side, and 

 the white color was shaded up and gradually mixed with the 

 brown of the sides. When viewed from a little distance these 

 two end ones, which were white below, disappeared from sight, 

 while the middle one stood out in strong relief and appeared 

 much darker than it really was. Mr. Thayer explained that 

 terrestrial birds and mammals which are protectingly colored 

 have the under parts white or very light in color, and that the 

 color of the under parts usually shades gradually into that of 

 the upper parts. This is essential in order to counteract the 

 effect of the shadow, which otherwise, as shown by the middle 

 potato, makes the object abnormally conspicuous and causes it 

 to appear much darker than it really is. Some of the wit- 

 nesses could liardly believe that the striking difference in the 



