182
J. A. C. NICOL
1 ft c. There is an early experiment concerning Abramis brama, whioh
were kept in failing daylight one afternoon until darkness, 8 period of
14 hr ; at the end of this time cones were found to be retracted (Ener
and Januschke, 1905). Attempts have been made to relate the courm
of retinomotor changes in salmon with changes in environmental
light-levels and fish behaviour. Ali (1969) has found that retinomotor
changes of juvenile Pacific salmon (various spp.) lag behind the
decrease of ambient light-levels occurring in nature (from 1 to 0-002 ft 0
in 20 min): this estimate was based upon the time required for da&
adaptation after exposure to light of 400 ft c. Under natural conditions;
of decreasing illumination during the evening, cones and pigment of
pink salmon began shifting towards the " dark " conditions at levela
below 1 ft c and movement was complete between
and
ft c.
The natural illumination fell from 1 to lo4 ft c in 45 min. In the laboratory retinae were in the " dark " condition after exposure for 90 min to
10-1 (cones) and 10-2 ft c (pigment). It is not clear from the descriptions what intensities the fish experienced before the experiment began
(Ali and Hoar, 1959). In Atlantic salmon yearlings, exposed to 0.1 ft 0,
the retinal pigment retracted after 45 min and the cones became
maximally elongated after 25 min. Retinomotor changes towards the
dark condition commence in this species when the light falls to 0.1 ft o
and are complete at an illumination of 0.0001 ft c. Environmental
illumination falls to this extent in about 45min (varying of course
with cloud, etc.) and it might be expected from the data. presented
above that retinomotor changes would keep pace with this dec.rrem
(Brett and Ali, 1958 ; Ali, 1961a, b, 1962).
Photometric data for levels of illumination in the environment,
and in experiments relating to retinomotor changes, have been PI%sented as foot candles, and both sets of measurements are comparable.
But other measurements relating to visual threshold, light-intensities
in the sea, animal luminescence, etc., are given in energy units, ergs/
sec, watts, or photons, and some day these various kinds of information will have to be collated and integrated. In view of the unspecified composition of the lights and illumination employed end
measured in the first kirds of investigation mentioned above, thia W k
may prove to be very difficult.
IV. REFRACTION, ACCOMMODATION AND TEE RECEPTOB LAYEE
It is generally believed that elasmobranchs are hypermetropic, and
that they accommodate for near vision by moving the lens towards the
cornea through contraction of a protractor lentis muscle. Teleosta, on
the other hand, are believed to be myopic, and to aocommodate for
J. A. C. NICOL
1 ft c. There is an early experiment concerning Abramis brama, whioh
were kept in failing daylight one afternoon until darkness, 8 period of
14 hr ; at the end of this time cones were found to be retracted (Ener
and Januschke, 1905). Attempts have been made to relate the courm
of retinomotor changes in salmon with changes in environmental
light-levels and fish behaviour. Ali (1969) has found that retinomotor
changes of juvenile Pacific salmon (various spp.) lag behind the
decrease of ambient light-levels occurring in nature (from 1 to 0-002 ft 0
in 20 min): this estimate was based upon the time required for da&
adaptation after exposure to light of 400 ft c. Under natural conditions;
of decreasing illumination during the evening, cones and pigment of
pink salmon began shifting towards the " dark " conditions at levela
below 1 ft c and movement was complete between
and
ft c.
The natural illumination fell from 1 to lo4 ft c in 45 min. In the laboratory retinae were in the " dark " condition after exposure for 90 min to
10-1 (cones) and 10-2 ft c (pigment). It is not clear from the descriptions what intensities the fish experienced before the experiment began
(Ali and Hoar, 1959). In Atlantic salmon yearlings, exposed to 0.1 ft 0,
the retinal pigment retracted after 45 min and the cones became
maximally elongated after 25 min. Retinomotor changes towards the
dark condition commence in this species when the light falls to 0.1 ft o
and are complete at an illumination of 0.0001 ft c. Environmental
illumination falls to this extent in about 45min (varying of course
with cloud, etc.) and it might be expected from the data. presented
above that retinomotor changes would keep pace with this dec.rrem
(Brett and Ali, 1958 ; Ali, 1961a, b, 1962).
Photometric data for levels of illumination in the environment,
and in experiments relating to retinomotor changes, have been PI%sented as foot candles, and both sets of measurements are comparable.
But other measurements relating to visual threshold, light-intensities
in the sea, animal luminescence, etc., are given in energy units, ergs/
sec, watts, or photons, and some day these various kinds of information will have to be collated and integrated. In view of the unspecified composition of the lights and illumination employed end
measured in the first kirds of investigation mentioned above, thia W k
may prove to be very difficult.
IV. REFRACTION, ACCOMMODATION AND TEE RECEPTOB LAYEE
It is generally believed that elasmobranchs are hypermetropic, and
that they accommodate for near vision by moving the lens towards the
cornea through contraction of a protractor lentis muscle. Teleosta, on
the other hand, are believed to be myopic, and to aocommodate for