156
predict the Q's factors for randomly oriented cylinders and disks, and also for two-layered
spheres, with absorbing material concentrated within the inner core or in the outer shell. The
Qc. Q. curves for such particles have in common with those for spheres the same limiting
values and a similar sequence of progressively diminishing maxima and minima. With respect
to curves for spheres, their positions are not directly comparable, to the extent that the "p"
parameter, unambiguously defined in the case of sphere, can be defined in different ways
when dealing with different shapes of particles. A representative p parameter for such nonspherical particles could be computed by introducing a "size" defined as being the ratio of the
volume to the mean geometrical cross section, V/S g , (Aas, 1984). To our knowledge,
experimental verifications of these theoretical findings for cylindrically shaped particles remain
to be effected. Experiments with various powders, crushed minerals, and other irregularly
shaped particles have confirmed the expected disappearance of oscillations in the Qb curve (see
also papers in "Light scattering by irregularly shaped particles" edited by D.W. Schuerman,
Plenum, 1980, where emphasis is put on aerosol particles with transferable results in the case
of marine particles, and also theoretical papers in "Electromagnetic scattering" edited by M.
Kerker, 1963).
The comparison made between theory and experiments in Figure 2 is restricted to a single
wavelength. With given index and size (or actually a range of sizes for a population of living
organisms) and when changing the wavelength, a large portion of the p domain can be
explored. In effect, p is multiplied by the ratio 750 / 400 when "A goes from 400 to 750 nm,
as a consequence of Eq. 7 and 9. Therefore, the comparison between theory and experiments
is possible over an extended p-range (Fig. 3), and this is actually the basis on which the
possibility of inferring the real part of the refractive index of biological particles rests (see
Bricaud and Morel, 1986 and discussion therein). Nearly non-absorbing bacteria exhibit a
regularly ascending Qb ("A) curve, from red to blue, which corresponds to the initial part of
the theoretical Qb curve in Fig. 2. Their actual Qb values fix the possible range of variations
in p and hence, (their size being known), the compatible n-value. The same reasoning cannot
apply when naked ciliates are considered. With their rather big sizes (13 - 18 JLm), they enter
into the p domain where Qb is equal to 2 and thus remains constant whatever the wavelength.
The refractive index for ciliates consequently remains undetermined. Conversely,
heterotrophic flagellates and the cyanobacterium Synechocystis belong again to the initial
predict the Q's factors for randomly oriented cylinders and disks, and also for two-layered
spheres, with absorbing material concentrated within the inner core or in the outer shell. The
Qc. Q. curves for such particles have in common with those for spheres the same limiting
values and a similar sequence of progressively diminishing maxima and minima. With respect
to curves for spheres, their positions are not directly comparable, to the extent that the "p"
parameter, unambiguously defined in the case of sphere, can be defined in different ways
when dealing with different shapes of particles. A representative p parameter for such nonspherical particles could be computed by introducing a "size" defined as being the ratio of the
volume to the mean geometrical cross section, V/S g , (Aas, 1984). To our knowledge,
experimental verifications of these theoretical findings for cylindrically shaped particles remain
to be effected. Experiments with various powders, crushed minerals, and other irregularly
shaped particles have confirmed the expected disappearance of oscillations in the Qb curve (see
also papers in "Light scattering by irregularly shaped particles" edited by D.W. Schuerman,
Plenum, 1980, where emphasis is put on aerosol particles with transferable results in the case
of marine particles, and also theoretical papers in "Electromagnetic scattering" edited by M.
Kerker, 1963).
The comparison made between theory and experiments in Figure 2 is restricted to a single
wavelength. With given index and size (or actually a range of sizes for a population of living
organisms) and when changing the wavelength, a large portion of the p domain can be
explored. In effect, p is multiplied by the ratio 750 / 400 when "A goes from 400 to 750 nm,
as a consequence of Eq. 7 and 9. Therefore, the comparison between theory and experiments
is possible over an extended p-range (Fig. 3), and this is actually the basis on which the
possibility of inferring the real part of the refractive index of biological particles rests (see
Bricaud and Morel, 1986 and discussion therein). Nearly non-absorbing bacteria exhibit a
regularly ascending Qb ("A) curve, from red to blue, which corresponds to the initial part of
the theoretical Qb curve in Fig. 2. Their actual Qb values fix the possible range of variations
in p and hence, (their size being known), the compatible n-value. The same reasoning cannot
apply when naked ciliates are considered. With their rather big sizes (13 - 18 JLm), they enter
into the p domain where Qb is equal to 2 and thus remains constant whatever the wavelength.
The refractive index for ciliates consequently remains undetermined. Conversely,
heterotrophic flagellates and the cyanobacterium Synechocystis belong again to the initial