World of the Dark 
far more delicately and instantaneously than any device, attached 
to the most perfectly constructed camera, regulates the amount 
of light transmitted through its lens. The tiger in the jungle 
sees what is going on about him in the starlight as well as we 
see what is happening in the noontide. I have studied the 
eyes of lions and tigers in the dark. The yellowish-green iris in 
the night almost entirely disappears from view, and shrinks 
down into a narrow ring. The windows of the eyes have the 
curtains drawn back wide, so as to let in all the light which the 
darkness holds within itself. The great orbs then look like 
globes of crystal, framed in a narrow band of gold, lying on a 
background of the blackest velvet, while in their pellucid depths, 
fires, tinged with the warm glow of blood, play and coruscate. 
The eyes of many birds are adapted to the dark. This is 
true, as everybody knows, of the owls, and of their not distant 
relatives, the goat-suckers. I remember having, when a boy, 
dissected an owl, which 1 found dead after a long protracted 
period of intensely cold weather. The thermometer had stood 
at twenty degrees below zero for several nights in succession. 
The earth was wrapped deep in snow. Upon the sleety crust 
I found a great horned owl, lying dead, and frozen stiff. It may 
have died of old age, or it may have starved to death. The 
instinct of the child, who takes his toys to pieces in order to see 
how they are made, seized me, and, with a sharp penknife as a 
scalpel, and a few needles set in sticks of pine, I took my owl 
apart, and made drawings of what I found. I did not then 
know the names and functions of all the parts, but the drawing 
of the eye, which I made, I still have in an old portfolio, and 
there I saw it the other day. The eye of an owl is a wonderful 
piece of mechanism. It is a wide-angle lens of beautiful powers 
of adjustment. It is adapted to taking in all the light there is, 
when the light is almost all gone; and it is so contrived as to 
shut out light, when too much of its splendor would dazzle 
and hurt. 
Among the insects thousands and tens of thousands of 
species are nocturnal. This is true of the great majority of the 
moths. When the hour of dusk approaches stand by a bed 
of evening primroses, and, as their great yellow blossoms 
suddenly open, watch the hawkmoths coming as swiftly as 
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