
Bob cat, possessed of keen nocturnal vision. 
DO YOU KNOW 
—that if you were endowed with the eyesight 
of an eagle, you could read this at 500 feet? 
—that an owl can “stop down” the diaphragm 
of its eye from the size of a finger ring to 
that of this letter op 
—that snails ‘“‘pull in’ their eyes? 
—_that a species of fish changes the position 
of its eyes with growth? 
Marvels of Bird and Animal Vision 
that the ordinary man, possessed of 
a highly developed sense of sight, 
hibitually presumes that other crea- 
tures see as he does and are in like 
manner familiar with form and color. 
It requires but a moment’s thought 
however to bring realization of the vast 
difference which must exist between the 
eyesight of insects and that of fishes, 
and it is obvious that the vision of 
animals which spend their days in semi- 
darkness must be far inferior to that 
of the high soaring birds of prey. Of 
recent years scientists have devoted 
considerable attention to this subject, 
with the result that many new ocular 
facts have been brought to light and 
added to the sum total of human knowl- 
edge. There is, nevertheless, much still 
to be discovered in regard to the fac- 
ulty of sight in birds, in animals and 
in the myriad forms of insect life pres- 
ent upon this earth. This is a branch 
of nature study which is unusually 
fascinating, for in such a field the pos- 
sibility of startling discovery is one 
that is ever present. 
The abnormally keen vision of hawks 
and eagles has since very remote times 
been noted by mankind. Few people 
however realize how highly developed 
is the eyesight of these predatory birds. 
A person blessed with normal vision 
can in daylight readily distinguish the 
letterpress on this page at a distance 
of approximately five feet. Had he 
the eyepower of an eagle it would be 
possible for him to read these lines at 
a distance of five hundred feet, a truly 
amazing feat. Soaring far up in the 
blue of heaven, the eagle scanning the 
earth’s surface can easily detect the 
650 
[' is a somewhat curious circumstance 
By DAN McCOWAN 
Photos by the Author 
tiny moving dot that represents a 
ground squirrel or a coney. Suddenly 
the great bird folds his wings and, 
dropping like a plummet, bears away 
the stricken victim in his talons. This 
meteoric headlong dive from cloudland 
to earth seems to occupy the merest 
fraction of time, yet during its course 
the eye of the bird is constantly fixed 
upon its intended prey. Further, as 
the eagle descends, the focus of the eye 
must be, and is, adjusted with a speed 
and an accuracy that is, to say the 
least, marvelous. 
Far removed, both bodily and in com- 
parative sense of sight, from the keen- 
eyed birds that circle through upper 
air, almost, but not quite at the other 
end of the ocular list, comes the Mole. 
Living in underground chambers, 
creeping along dark earthy tunnels, 
thrusting its body through soil and 
sand, it is but natural that the eye of 
the mole should be both small and but 
poorly developed. In most animals of 
this species the diameter of the eyeball 
does not exceed one twenty-fifth of an 
inch. This tiny eye is complete in 
every detail of iris, lens and retina, but 
its lack of power is such that the ani- 
mal is to all intents and purposes blind. 
@)a species of mole found in south- 
ern Europe has a thin membrane 
over the eyes probably to protect these 
delicate organs from gritty particles of 
earth. This membrane, although 
pierced by small holes, obscures the vi- 
sion to such an extent that while the 
animal is able to discern light from 
darkness it has no distinct vision. 
In birds and beasts that are largely 
nocturnal in habit the eyes have been 
wonderfully modified to suit their spe- 
cial needs. No creature can see in to- 
tal darkness: that is impossible. But 
such night frequenters as owls and cats 
are able to detect and capture their 
prey when there is an almost entire 
absence of light. 
| Pe eye of the owl is an unusually 
complex organ, the form being 
tubular, the lens rounded, the pupil 
widening or narrowing at will of the- 
bird. It is interesting to observe an 
owl “stopping down” the diaphragm of 
his eye from an opening equal to that 
in an ordinary finger ring to that of 
a small “o” as printed on this page. 
The eye of this and other types of night 
flying birds are as a rule focused for 
normal near vision. There is a popular 
and widespread belief that owls can- 
not see in daylight. That of course 
is not so. Owls may not have clear 
vision when in bright light, but they 
can see well enough to move from tree 
to tree and have been observed to pick 
up small pieces of meat when there 
was sunshine glare. 
If the eye of a cat be exposed to 
strong light the pupil will at once con- 
tract until it appears as a narrow ver- 
tical slit. In weak light it becomes 
dilated to an extraordinary degree. By 
opening the pupil to its fullest extent 
faint rays of light are caught and 
transferred to the optic nerves. As a 
further aid the eyeball of the cat is 
furnished with lining of a brilliant yel- 
low color which serves to reflect and 
intensify such feeble glimmerings of 
light as may be present during the 
night hours or in darkened places. 
Noteworthy amongst mammals havy- 
