The Cairo bulletin., 1909-08-24
EYES OF MEN, BIRDS AND IN.
SECTs.

Mans eyes at rest are far focused
will make no effort when seeing the
moon or earthly horizons. Birds and
fishes eyes at rest are near focused
will make no effort when looking at
nearby worms and minnows. Mans
| elastic lenses are under constant nat
| tening compression. Imagine a rub
shape. laid In between two disks OF
canvas, and the uniting edges 81 these
cloths. stretched to Il ring. They
would tiatten the rubber and II re-
Taxed it would thieken by its own
elasticity. The thicker the lens the
shorter It's focus. For reading or
threading d needle we relax the ten
sion of the lens by contracting ring
Of muscle surrounding each lens, and
then wait for the lenses TO thicken
through their elasticity In fish the
lens is set against the cornea zap
proximately) short focus, and when it
wants to see whether the shadowy
object some feet away is shark or
log. 1t pulls the entire round lens to-
ward the retina, and gets as clear
vision as possible Now we see why
so many human beings need spec
taeles" as they grow oId-the elastic
ity of the lenses is gradually lost, just
as IL 10 in rubber One more method
of getting focus i employed by the
eyemaker, which Us dealt 10 some
snakes. Their lenses. which are set
near the retinas. are pushed forward
after the manner OF pump piston, by
blood pressure, Cheap eyes for cheap
creatures, Focus regulated by excite
ment
Some of the eyes that can see in
the dark have no power OF changing
focus: so it makes no difference
whether they get night edition OF
the days doings or not There is
prevailing they say" opinion that
birds sight is keener than mans
This is probably not true, as only man
and the simiae. which have parallel
vision, possess a highly coucentrat-
ed sensitive area in the retinas-the
macula lutea.
But birds aloft are in clearer air
than man, and their eyes can change
focus with remarkable speed, as
necessitated by rapid flight Birds OF |
prey have voluntary Subject to the |
will) musae as we'll as involuntary
In their irides. and can increase the |
eonvexity OF the eornea and its re- |
fractive power. Who has not wond-
cred how sparrow-hawk could dart |
through brush and trees aud never |
turn a feather by collision? Man with |
his flying machines will be iu sore
need Of bird focusing speed
We can see the single eyes OF some
insects without lens, as in the lo-
Gust. In viewing the house-fy we |
need a lens. The big, visible, bulg- |
ing eyes we see are composed of thou /
sands of unit, cone-shaped eyes bound |
into one compound eye each, of more
or less spherical shape Under lens
they look like glass-eyed pavement |
bent 10 conyexity. Their faeeted cor
nshs are variousIy set in square, hexa-
gonai or prismatic frames. Each
glistening facet is the eornea lens Of
a distinct self-gorking eye. Their
number In each compound eye is enor-
nous. There are 50 such eyelets in
each IN the ant, 400 are allowed the
irone bee. and $500 the workers"
Dur pet kitchen fly has 5000 chances
If seeing food crumbs, the beetle over
1000, while more than $4,000 aid the
in his eleemosynary pur-
suit of the mosquito, offset somewhat
ny several thousand awarded the lat
er for sporting chance" The hawk
moth gets pictures compounded by
%. 900 contributors. Over $4,000 win
low the brain of the mordella (beetle)
md 60.000-so it is elaimed-contrib-
tie to the happy lives of some butter
lies.-Dr Edward A. Ayres in Har-
Jer's Magazine.
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