marks the sand glinting through the moving water. Again, the system of 

 white and golden marks together simulates the flickering sun stripes on the 

 bottom, made by refraction from the ripples. Naturally, the life and realism 

 of these pictures are greatly enhanced by the iridescence of the green ground- 

 color. 



There are a great many other beautiful cases of this use of iridescence in 

 aid of definite background-picturing, but the above example must sufiice us 

 here. One more small detail, however, one more phase of the use of change- 

 able color, must be described. It is one to which I have already alluded, in 

 part, in this and an earlier chapter, namely, the apparent 'opening of win- 

 dows' in a dull-colored surface by the application of bright spots and stripes. 

 The brightest iridescent and sheenless changeable colors are often set in 

 spots like jewels in an otherwise dull costume. Common and important in " 

 the case of birds, this type of coloration is even more so in that of butterflies. 

 But these will be considered later, and we are here concerned with birds alone. 

 Many birds, particularly tropical ones, have such gemlike spots in the midst 

 of somber plumage. Often they are surrounded by dead black, or some 

 very dark tone of brown or gray. This encompassing dusky pattern, being 

 usually quite lusterless, is the same in all lights, while the bright spot in its 

 midst flashes and alters with every little shift of light or movement of the 

 bird or the beholder. Therefore it has the look of a hole in a motionless 

 dark obstruction — a glimpse through a somber shadow — ^beyond which are 

 seen sky vistas or the flickering light and movement of vegetation. Or, again, 

 the bright spots may pass for moving bits of vegetation relieving against a 

 motionless shadow or hole behind them. In either case, the solid form of the 

 bird will be effectually 'cut to pieces.' 



To sum up: changeable colors of all sorts strongly tend to conceal the 

 birds that wear them, and iridescence is extraordinarily potent in this way. 

 Its power is of two kinds, which are, however, practically inseparable in their 

 working. First, it goes far toward annulling the normal lights and shadows, 

 with their color-effects, of the surface on which it is placed; and second, its 



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