SOME ASPECTS OF PHOTORECEPTION AND VISION I N FISHES
183
distance. The teleost eye contains a retractor lentis muscle (wmpanula
HaUeri), contraction of which displaces the lens backwards along the
axis of the fish’s body and more or less inwards, according to the species.
Hypermetropia in elasmobranchs, according to Franz, amounts to 10
to 15 dioptres, and they can accommodate from 15 to 20 dioptres.
Verrier, moreover, found that hypermetropia was also characteristic
of the eyes of teleosts, even when accommodated (references in Baylor
and Shaw, 1962). Hypermetropia wm detected in the eyes of a wide
variety of fishes by Baylor and Shaw (1962). Some mean values for
refractive error were 8 dioptres in alewife (Absa psezldoharengue) and
14.6 to 15.8 in silversides (Menidia menidia). Dogfish and skate had a
hypermetropia of 6 and 8 dioptres. Accommodation in some of the
teleosts they examined amounted to 6 to 10 dioptres. In A h a the
refractive error in a line antero-posterior (main axis) waa lower than in
lateral view (3.6 vs 8.4 dioptres). The eye of the alewife is ellipsoidal,
and is slightly longer (by 10%) in the antero-posterior axis than in the
minor optic axis, from lens to centre of the fundus.
A most useful treatment of vision and lens functioning in the normal
fish eye has been offered by Pumphrey (1961). The lens is spherical and
its dioptric strength is high, ranging, for example, from several thousand
dioptres in a juvenile fish a few inches long to several hundred in an
adult 1 ft long. The focal length is short, and measurements of refraction
have shown that there is a constant ratio of focal length to radius having
limits of 2.5 to 2.6 ; this ratio is named after Matthiessen, its discoverer.
It is obvious that a refractive error of, say, 10 dioptrea in 1000 is quite
small, and amounts to a displacement of focal length of only loop.
Unfortunately, it is far from certain what refleoting surface is being
examined in retinoscopy of the fish eye ; Beer thought that retinorrcopio
reflexion occurred at the front of the retina ; Baylor and Shaw (op cil.)
from behind. The retinal pigment layer itself amounts to almost loop
in eyes of salmon fry, the retina inside the external limiting membrane
is 200p thick, and rods, cones and pigment make considerable rrtdiaJ
excursions. It still has to be demonstrated, convincingly, that the plane
of focus in the eye at rest lies elsewhere than in the receptor layer;
the so-called error may be one not of refraction, but rather of method.
The fish lens is virtually aplanatic and its focal length is remarkably
short (2.5 x radius). It is argued that a lens having these characteristics
and made of biological material whose refractive index cannot exceed
1.53, must have an index that decreases from a maximum of 1-53 at
its centre to 1.33 at its periphery, where the index approximatea that
ofthe surrounding medium. This change of index is difficult to measure,
but it is believed to be the only explanation that fits the facts.
183
distance. The teleost eye contains a retractor lentis muscle (wmpanula
HaUeri), contraction of which displaces the lens backwards along the
axis of the fish’s body and more or less inwards, according to the species.
Hypermetropia in elasmobranchs, according to Franz, amounts to 10
to 15 dioptres, and they can accommodate from 15 to 20 dioptres.
Verrier, moreover, found that hypermetropia was also characteristic
of the eyes of teleosts, even when accommodated (references in Baylor
and Shaw, 1962). Hypermetropia wm detected in the eyes of a wide
variety of fishes by Baylor and Shaw (1962). Some mean values for
refractive error were 8 dioptres in alewife (Absa psezldoharengue) and
14.6 to 15.8 in silversides (Menidia menidia). Dogfish and skate had a
hypermetropia of 6 and 8 dioptres. Accommodation in some of the
teleosts they examined amounted to 6 to 10 dioptres. In A h a the
refractive error in a line antero-posterior (main axis) waa lower than in
lateral view (3.6 vs 8.4 dioptres). The eye of the alewife is ellipsoidal,
and is slightly longer (by 10%) in the antero-posterior axis than in the
minor optic axis, from lens to centre of the fundus.
A most useful treatment of vision and lens functioning in the normal
fish eye has been offered by Pumphrey (1961). The lens is spherical and
its dioptric strength is high, ranging, for example, from several thousand
dioptres in a juvenile fish a few inches long to several hundred in an
adult 1 ft long. The focal length is short, and measurements of refraction
have shown that there is a constant ratio of focal length to radius having
limits of 2.5 to 2.6 ; this ratio is named after Matthiessen, its discoverer.
It is obvious that a refractive error of, say, 10 dioptrea in 1000 is quite
small, and amounts to a displacement of focal length of only loop.
Unfortunately, it is far from certain what refleoting surface is being
examined in retinoscopy of the fish eye ; Beer thought that retinorrcopio
reflexion occurred at the front of the retina ; Baylor and Shaw (op cil.)
from behind. The retinal pigment layer itself amounts to almost loop
in eyes of salmon fry, the retina inside the external limiting membrane
is 200p thick, and rods, cones and pigment make considerable rrtdiaJ
excursions. It still has to be demonstrated, convincingly, that the plane
of focus in the eye at rest lies elsewhere than in the receptor layer;
the so-called error may be one not of refraction, but rather of method.
The fish lens is virtually aplanatic and its focal length is remarkably
short (2.5 x radius). It is argued that a lens having these characteristics
and made of biological material whose refractive index cannot exceed
1.53, must have an index that decreases from a maximum of 1-53 at
its centre to 1.33 at its periphery, where the index approximatea that
ofthe surrounding medium. This change of index is difficult to measure,
but it is believed to be the only explanation that fits the facts.