EYES AND CAMERAS. Ill 



that they have inside them a little folded membrane called the 

 pecten, which our eyes lack. Its use is not known, but it has 

 been suggested that it helps to focus the eye instantaneously, 

 so that the hawk, swooping from a height, or the gannet, diving 

 from the air like an arrow, may always keep a clear and un- 

 bliirred vision of its prey. Our own eyes could not be adjusted 

 for such rapid motion. It is interesting to compare the elon- 

 gated eyeball and round lens of the nocturnal owl with the 

 flattened eyeball and lens of the diurnal hawk. Contrary to 

 popular opinion, the owl can see very well by day also, because 

 his eye is capable of great adjustment to the amount of light, 

 and can either collect the scattered, feeble rays of dusk and 

 darkness, or exclude the strong glare of day. 



Eyes are not only the most perfect cameras, but they are also 

 the smallest. Small as our own eyes are, there are others far 

 tinier. Think of the birds about us, the swallow chasing the 

 fly, the vireo tripping along a bough, the chickadee clinging to 

 a twig, searching for food too small for us to discover. Probably 

 the smallest cameras known are the eyes of the humming-birds. 

 The tiny Princess Helena humming-bird of Cuba is only two 

 and a quarter inches from bill-tip to tail-end, and its eye is 

 about the size of the head of a round-headed black pin. Can 

 there be a smaller camera than this ? But the little humming- 

 birds, when they first open their eyes, are not nearly as large 

 as their mother, and yet their eyes are as perfect as hers. 

 Surely these are the smallest cameras in all the world. 



