184
J. A. C. NIUOL
The typical teleost eye is semi-ellipsoidal, about 3 to 10% longer in
its antero-posterior axis than in the vertical or oornea-fundus axis and
the lens is a concentric sphere. Let it be assumed that images of far
objects are in focus on the anterior and central fundus when the lens ir
at rest, i.e. that these regions are emmetropic. Ths fish is then myopio
for forward vision and when it accommodates by moving the lens
backwards, it brings distant objects ahead into focus. Since the lens is
displaced along an antero-posterior axis, emmetropia in lateral view is
unaffected, but the range of dear vision behind the fish is simultaneously
shortened.' The extenb to which the retractor lentis muscle is developed
variea in different speoies; possibly the degree of accommodation is
correlated with habits. The situation in elagmobranchs is still obaonre
because both the retina and lens are slightly ellipsoidal, and the lens is
protracted.
Many nektonic teleost fishes possess adipose eyelids, which lie over
part or all of the eye ; when the eye is only partially covered the eyelids
leave a vertical aperture over the pupil. Adipose eyelids are regarded
as structures which enhance stream-lining (Walls, 1942). They are
usually clear fleshy structures consisting of densely arranged parallel
fibres which are oriented dorso-ventrally. Because of their structure
they are strongly birefringent and act aa an analyser for polarized light.
The amount of light they absorb varies with the plane of polarization
of the light, and some 40% less light is transmitted when the plane of
polarization is parallel to the dorso-ventral axis than when it is parallel
to the antero-posterior axis. The adipose eyelids thus provide a meass
by which fish can detect polarized light, at least in the peripheral field
(Stewart, 1962). The possibility that this faculty may be utilized in
the behaviour of fish has not escaped notice ; depending upon refkaotbn
indices, the eyelih may also affect the focusing power of the eye.
A fovea is rare in fish, the outer segments of the rods and mnea are
fairly long and a considerable depth of focus is consequently to be
expected. Cones in the fish eye tend to be large, well spread out tmd
arranged in mosaic patterns ; because of these factors the quality of an
image, in focus, may be much inferior to that perceived by the human
eye in air. In addition, the mattering properties of water reduce the
ability to resolve images.
The structure and arrangement of cone8 in gadoid fishes have been
described by Engstrt)m (1961), who haa found an " accessory element "
connected with the outer segment. Intraspeoific differences have been
reported in the retinae of Sebmtw wrinua, a benthic scorpion fish from
deep water (Hanyu and Ali, 1962). In a specimen of S.m. menteEIa from
275 m, cones were longer than in a specimen from 549 m; rode w m ~
J. A. C. NIUOL
The typical teleost eye is semi-ellipsoidal, about 3 to 10% longer in
its antero-posterior axis than in the vertical or oornea-fundus axis and
the lens is a concentric sphere. Let it be assumed that images of far
objects are in focus on the anterior and central fundus when the lens ir
at rest, i.e. that these regions are emmetropic. Ths fish is then myopio
for forward vision and when it accommodates by moving the lens
backwards, it brings distant objects ahead into focus. Since the lens is
displaced along an antero-posterior axis, emmetropia in lateral view is
unaffected, but the range of dear vision behind the fish is simultaneously
shortened.' The extenb to which the retractor lentis muscle is developed
variea in different speoies; possibly the degree of accommodation is
correlated with habits. The situation in elagmobranchs is still obaonre
because both the retina and lens are slightly ellipsoidal, and the lens is
protracted.
Many nektonic teleost fishes possess adipose eyelids, which lie over
part or all of the eye ; when the eye is only partially covered the eyelids
leave a vertical aperture over the pupil. Adipose eyelids are regarded
as structures which enhance stream-lining (Walls, 1942). They are
usually clear fleshy structures consisting of densely arranged parallel
fibres which are oriented dorso-ventrally. Because of their structure
they are strongly birefringent and act aa an analyser for polarized light.
The amount of light they absorb varies with the plane of polarization
of the light, and some 40% less light is transmitted when the plane of
polarization is parallel to the dorso-ventral axis than when it is parallel
to the antero-posterior axis. The adipose eyelids thus provide a meass
by which fish can detect polarized light, at least in the peripheral field
(Stewart, 1962). The possibility that this faculty may be utilized in
the behaviour of fish has not escaped notice ; depending upon refkaotbn
indices, the eyelih may also affect the focusing power of the eye.
A fovea is rare in fish, the outer segments of the rods and mnea are
fairly long and a considerable depth of focus is consequently to be
expected. Cones in the fish eye tend to be large, well spread out tmd
arranged in mosaic patterns ; because of these factors the quality of an
image, in focus, may be much inferior to that perceived by the human
eye in air. In addition, the mattering properties of water reduce the
ability to resolve images.
The structure and arrangement of cone8 in gadoid fishes have been
described by Engstrt)m (1961), who haa found an " accessory element "
connected with the outer segment. Intraspeoific differences have been
reported in the retinae of Sebmtw wrinua, a benthic scorpion fish from
deep water (Hanyu and Ali, 1962). In a specimen of S.m. menteEIa from
275 m, cones were longer than in a specimen from 549 m; rode w m ~