248
KENNETH V. THIMANN AND GEORGE M. CURRY
exposure. Since this was first studied by Bunsen and Roscoe for photochemical reactions, the dependence of phototactic response on this product is often referred to as the Bunsen-Roscoe Law, or simply the I X t
Law. The researches of Blaauw established this law for phototropism
of plants, under certain conditions (see Section IV, A). Loeb and
Wasteneys (11) tested it with the phototropic curvature of the hydroid
Eudendrium, by counting the number of positive curvatures developing
in the dark after a fixed exposure; they found it to hold for three intensities. Loeb and Northrop (12) used a method for phototaxis in
which two lights are placed before the animals, one being obscured to
a variable extent by a rotating sector-disc. The angle of movement of
Balanus nauplii between the two light sources was then found to be
proportional to the duration of illumination in the second source, provided that the ratio of illumination times was not greater than 10:1.
Such experiments with continual intermittent exposure may not necessarily be compared with single exposures of varying duration like those
used by Blaauw, since they may allow of recovery or reversion in the
photosensitive system during the dark period (see, e.g., Dolley, 13).
Since Loeb, the validity of the I X t law has been established, though
also within marked limitations, for a number of animals (see Rose 6).
In insects it appears to hold only for short exposures and low intensities
(14). While the initial photochemical reaction is doubtless proportional
to the total incident light energy (i X t), so many steps must intervene
between it and the resulting movement that it is probably naive to
expect any close correspondence.
(4) Spectral sensitivity. Study of the sensitivity to wavelength is
one of the best means of establishing the nature of the photoreceptor.
Its use in vision is discussed in Chap. 7 in this volume. Unhappily, the
technical difficulties involved in producing light beams of equal energy
at different wavelengths, as well as the imperfections of many color
filters, combine to render most of the early work on the spectral sensitivity of phototaxis open to serious doubt. However, a general insensitivity to the red has been noted in a variety of animals. Omitting a
number of conflicting or improbable data, the following regions of
maximum sensitivity seem to be established:
Yellow to yellow-green
Balanus larvae, Euproctis caterpillars
Blue-green
Palaemonetes, Loligo
503 τημ
Blowfly larvae
Blue
Cyclops, Chydorus, Diaptomus, and Bufo
species (both eyes and skin), probably most amphibians
483 or 495 τημ
Arenicola larvae
483 τημ
Lumbricus, Phacus
KENNETH V. THIMANN AND GEORGE M. CURRY
exposure. Since this was first studied by Bunsen and Roscoe for photochemical reactions, the dependence of phototactic response on this product is often referred to as the Bunsen-Roscoe Law, or simply the I X t
Law. The researches of Blaauw established this law for phototropism
of plants, under certain conditions (see Section IV, A). Loeb and
Wasteneys (11) tested it with the phototropic curvature of the hydroid
Eudendrium, by counting the number of positive curvatures developing
in the dark after a fixed exposure; they found it to hold for three intensities. Loeb and Northrop (12) used a method for phototaxis in
which two lights are placed before the animals, one being obscured to
a variable extent by a rotating sector-disc. The angle of movement of
Balanus nauplii between the two light sources was then found to be
proportional to the duration of illumination in the second source, provided that the ratio of illumination times was not greater than 10:1.
Such experiments with continual intermittent exposure may not necessarily be compared with single exposures of varying duration like those
used by Blaauw, since they may allow of recovery or reversion in the
photosensitive system during the dark period (see, e.g., Dolley, 13).
Since Loeb, the validity of the I X t law has been established, though
also within marked limitations, for a number of animals (see Rose 6).
In insects it appears to hold only for short exposures and low intensities
(14). While the initial photochemical reaction is doubtless proportional
to the total incident light energy (i X t), so many steps must intervene
between it and the resulting movement that it is probably naive to
expect any close correspondence.
(4) Spectral sensitivity. Study of the sensitivity to wavelength is
one of the best means of establishing the nature of the photoreceptor.
Its use in vision is discussed in Chap. 7 in this volume. Unhappily, the
technical difficulties involved in producing light beams of equal energy
at different wavelengths, as well as the imperfections of many color
filters, combine to render most of the early work on the spectral sensitivity of phototaxis open to serious doubt. However, a general insensitivity to the red has been noted in a variety of animals. Omitting a
number of conflicting or improbable data, the following regions of
maximum sensitivity seem to be established:
Yellow to yellow-green
Balanus larvae, Euproctis caterpillars
Blue-green
Palaemonetes, Loligo
503 τημ
Blowfly larvae
Blue
Cyclops, Chydorus, Diaptomus, and Bufo
species (both eyes and skin), probably most amphibians
483 or 495 τημ
Arenicola larvae
483 τημ
Lumbricus, Phacus