The sun. [volume] (New York [N.Y.]) 1833-1916, 1903-08-30
EYES THAT SEE IN THE DARK.
SPECU on THE SUBJECT
w DIR. AUSTIN FLINT.
Differences Between the Human Eye and
the Eyes ar Anlmais--DO the Srnsi-
tive Netlnas OF Nocturnal Beasts
Catch Light Rays Invisible 10 Qs::'


DR. Austin Flint has written the following
| acticle for Orr Animal Friends. which if
conducted by HI daughter
! Nearly every r~pdern treatise on physiology
begins with a description OF the ameba ibis
animal s a single. simple cell, capable OF
| various movements and changes of form.
and Of nourishing itself by the appropriation
! Of surrounding matters. II ls the simplest
example Of independent animal life, and Ia
taken az the type of animal cells belonging
to higher organisms, The development Of
| mammals. and indeed Of all animals. inClud-
ing man, begins in n single simple cell almost
exactly like the aneba. but with certain
coverings which serve not for its Develop
mcnt, but for protection and for It's attach
ments. This simple cell. united with fecund-
| ating organism OF m croscopio size, UNDER
goes multiplication by process that * called
indirect cell-division. The original cell II
about one-hundredh OF an inch in diameter.
In the process Of development the daugh-
ter cells. resulting from the division Of the
original cell and the redvision and continued
| redivision of the daughter coils. build up
finally the perfect creature The cells which
make up the new being gradually become
specialized ns to structure and function
some become hard by the appropriation of
calcareous matter. and form bones; some
become very contracte. and form muscles
some float In the blood and carry oxygen
To all parts. some form tho nervous system.
which establishes relations with the external
world and with other organisms regulating
as well the various internal functions among
those which establish what are called the
functions of relation are cells so specialized
as to give The sense of sight or vision.
In the animal S Gale it is not found that
special nerves and organs are developed
beyond the natural requirements of individual
species. Specialization provides for division
Of function A mollusc does not need and
does not have rerfnal eye. A dog or a cat
needs no part for the exact appreciation of
small details OF objects and liz not provided
with an eye that would enable it 10 read.
for example, but many inferior animals have
requirements not necessary to man. Birds
of prey can distinguish relatively small OBJ
iects at immense distances when they would
be invisible to the unAidcd eye of man: but
human intelligence has produced the tee-
scope by means Of which We can see further
and better than birds. Nocturnal animals
certain insects. reptiles fishes. birds and NAM
mails, including horses and cats can see under
conditions of comparative obscurity, or in
what we call the dark, although il is difficult
to conceive of vision in total absence of light ::
but man can provide artificial light, When
organs OF special sense in man are inferior
to similar organs in lower animals. this defect
is more han supplied by superior int elieence
as in the construction of aids to vision. and
often man makes Use of superior special
senses in the lower animals. as. for example
the sense of smell in dogs
In some of the very lowest orders OF ami
mails. as the hydras which are little above
the amebas- the faculty of appreciation of
luminous rays is not localized. hut il ia prob
able that light produces some impression on
the general surface although this can hardly
be called vision The color bearing vesiCles
acknowledgement of Chameleons are pcCu-
liary sensitive to blue-v violet rays and have
nervous connections with the bra in. II is
possible thut these vesiCles. which are change
able in color may be concerned in sort of
cutaneolls vision for, by reflex action through
connections with ncrve-centrcs. and possibly
by voluntary action. the cells may be mad
20 change their color. In certain pcctens
lsCallopsl there arc little Glohular bodies
attached each by cylindrical pcdicle to the
borders Of the mantle, which are Thought ia
serve as non-retin0l eyes, and, indeed are
called ny naturaliSts oCuiform tuberccs, or
ocular pe,luncles These are found in spon-
dyles and in many other varieties OF mol
Mscs: but they are quite different from the
cephaiC eyes OF the typical mousca
The most important difference between
the human eve and the eyes of all other nam
mails except monkeys is in the prrsenc" in
man Of the rovea Centralis. an ovOd cup
shaped depression in the retna. living exactly
in the axis of direct vision, This little de-
preston is hardly more than one-liftnth
Of an inch in diameter tSchultHoD In this
part the anatomical elements of the Ie,ina
which receive visual impressions are never
stained hv what is called visual purple or
visual yellow. This Is the only part of the
retina that ia used in near vision. IA in read
ing. 1t exists IN monkeys animals that are
supposed 10 be the immediate ancestors of
man
Impressions made upon the fovpa to be
fully and exactly appreciated, require bright
illumination OF objects. The area of direct
vision. indeed. is practically useless in OBJ
scurty. In near vision tho pupil is con-
tracted, shutting on a great pnrt of the light
from the general surface of the Tetina the
black layer OF the Letina absorbing the ak
ing its curvHtures 80 that I'll" image is exactly
focused. In fact. the area OF direct and
distinct vision is used by day md the larger
area of indirect. or pcriphcral vision by
night, but Of course. use the area of
periphcrul vision also by day for general
appreciation of form. distance, outline and
color 1t ls evident that the lower animals
can have little or no use for distinct and OK
elusive area of direct vision: still if we watch
monkeys and see them hold an oh iect and
examine IT closely with almost human Curi-
osty. il suggests visual apparatus almost
identical with the perfect human eye. The
small extent OF the area Of direct vision is I'M
doubtndly useful; Tor were II larger. il is
probable that the MUD would become con
fused by the extent and variety of impress
siOns, aud il would not be 50 easy TO observe
minute details and fix the attention upon
small objects
The area of indirect or periphrral vision
u in the form Of an irregular ovoid extending,
Tor whit" light. about 90 degrees externally
from the visual nNis and about 10 degrees
internally and vertically above and below.
ColorCd, produce n LIQUID which stains The
cl"menls Of the retinN that are sensitive to
light. This C0loring matter forms in the
dark and is gradually hlcnchcd nv the act on
of light Under ordinary exposure to day
light II becomes yellow. IL il called visual
purple or visla, yellow. IN the dark, aflcr
cNposur" To light. the intense purpIr color
is restored, and tn" shades of color. between |
Pnrnle and yellow. regulate the -enSihitv !
OF tn" eye to light When we go suddenly |
from dark room into a brilliant light, the |
eyes are daZHled, the light is even painful |
ana vision IL very imperfect and when We |
pass from bright light into comparative OBJ |
scrutiny, Tor some moments. until the purple |
IL restored, vision is impossible. The change
from purple. TO yellow is fairly rapid and |
he eye readily accommodates itself to diff
from yellow 10 white il difficult and slow aud |
occurs only after exposure to "a glaring light |
or to the direct rays of the sun. So far NJ
- known, visual purple exists in all animals |
possessed OF retinnl eyes.
NJ, term nNrlalopa. applied TO the lower