88 METEOROLOGICAL OPTICS 
largely to the solution of the problem of light scatter- 
ing in the atmosphere. 
Unsolved Problems and Future Research 
As was shown in the preceding sections, much valu- 
able information can be obtained from a comparison of 
measured and theoretical values. Therefore future re- 
search depends primarily upon the development of 
both experimental and theoretical methods. First, there 
is a definite need for better equipment for measuring 
skylight polarization. Such equipment should permit 
objective, fast, and precise measurements of the degree 
of polarization and of the position of the plane of polar- 
ization, in comparatively narrow spectral ranges, in all 
directions over the sky, with special provision for meas- 
urement of weak polarization, thus being adaptable also 
to the measurement of the neutral points. Speed im 
measurement is required not only for almost simultane- 
ous measurements at different wave lengths and at dif- 
ferent places in the sky, but mainly for the possibility 
of recording the shorter fluctuations observed in the 
degree of polarization [57] as well as in the position of 
neutral points [45, 46]. These fluctuations, not observ- 
able by the older methods or eliminated by improper 
smoothing, seem to precede cloud formation in a clear 
sky, and might be used in the study of condensation 
processes in incipient clouds. With sensitive photomul- 
tipliers and with a proper amplification, the sensitivity 
of measurement can be increased beyond that of the 
human eye, and observations can be extended far into 
the twilight or used for optical sounding by searchlight 
beams. 
From the theoretical standpoint, Chandrasekhar’s 
computations can easily be completed, and skylight 
polarization due to multiple molecular scattermg can 
be computed, in all directions and for all wave lengths, 
for larger solar elevations when the atmosphere can be 
considered as plane-parallel. But the greatest advantage 
of this method is that it can be extended in such a way 
that almost all currently unsolved problems of atmos- 
pheric polarization can be solved. In the case of reflec- 
tion at the ground, if, for example, the polarization 
according to Fresnel’s law were taken into considera- 
tion as observed in the vicinity of large water surfaces 
[26], the anomalies mentioned earlier could be explained 
theoretically. If the Mie-Debye theory of large-particle 
scattering could be simplified in such a way that the 
corresponding scattering matrices could be computed 
without essential difficulties, the effect of large particles 
could also be taken into consideration. So far Chan- 
drasekhar’s theory has been limited to a plane-parallel 
atmosphere, and twilight anomalies have been unac- 
cessible to theoretical investigation of similar type. 
Chandrasekhar [14, 15] already has shown the way to 
extend the theory for a spherical atmosphere. It is nec- 
essary only to work out the suggested method in more 
detail. The exact theory of twilight phenomena cannot, 
however, be developed without the consideration of re- 
fraction and the corresponding bending of light paths 
in the atmosphere. This part of the problem does not 
represent any special difficulty since complete refrac- 
tion tables, with all the parameters necessary for such 
a computation, are now available and can be con- 
veniently used for this purpose [40]. 
Hence, many problems quite accessible by modern 
facilities are open to further investigations. In the re- 
view given above, the list of problems to be solved is 
not exhaustive; new problems may easily arise as the 
study of skylight polarization progresses further. For 
this reason atmospheric polarization deserves more 
attention than it has received up to now. 
REFERENCES 
A detailed discussion of problems of skylight polarization 
will be found in references [1] through [5]. Reference [6] gives a 
detailed description of instruments used in polarization meas- 
urements. Other references cited in the text follow. 
1. Busco, F., und Jensmn, C., ‘““Tatsachen und Theorien der 
atmospharischen Polarisation.” Jb. hamburg. wiss. Anst., 
Bd. 28, (1910), Berlin, Duemruler, 1911. 
2. JENSEN, C., ‘Die Himmelsstrahlung,”’ Handbuch der Phy- 
sik, Bd. 19. Berlin, Springer, 1928. (See pp. 70-152) 
3. —— “Die Polarisation des Himmelslichtes,’? Handbuch 
der Geophysik, Bd. 8. Berlin, Gebr. Borntrager, 1942. 
(See pp. 527-620) 
4. Linxe, F., ‘‘Die Theorie der Zerstreuung, Extinktion und 
Polarisation des Lichtes in der Atmosphiare,”’ Handbuch 
der Geophysik, Bd. 8. Berlin, Gebr. Borntrager, 1942. 
(See pp. 120-238) 
5. Pernter, J. M., und Exner, F. M., Meteorologische Optik, 
2. Aufl. Wien-Leipzig, Braunmiiller, 1922. (See pp. 644-— 
725) 
6. Jensen, C., “Die Apparate zur Untersuchung der at- 
mosphiarischen Polarisationserscheinungen,” in KiEINn- 
scumip?, E., Handbuch der meteorologischen Instrumente. 
Berlin, Springer, 1935. (See pp. 666-693) 
7. Autertmm, F., ‘“‘Zur Theorie der atmospharischen Polari- 
sation.”” Jb. hamburg. wiss. Anst., 32:1-66 (1915). 
8. Barprer, D., “Sur la correction de diffusion dans les 
mesures d’altitude des couches atmosphériques émettant 
la lumiére du ciel nocturne.’’ Ann. Géophys., 1:144-156 
(1944). 
9. BricxHan, F., ‘‘Vergleichende Messungen der Polarisation 
des Himmelslichtes in Frankfurt a. M. und am Taunus- 
observatorium.”” Beitr. Geophys., 42:208-227 (1934). 
10. Buumer, H., ‘‘Strahlungsdiagramme kleiner dielektrischer 
Kugeln.’”? I—Z. Phys., 32:119-134 (1925); I1—Ibrd., 38: 
304-328 (1926). 
11. Buscs, F., ‘“Beobachtungen tiber die atmospharisch-op- 
tische Stérung des Jahres 1912.”’ Meteor. Z., 30:321-330 
(1913). 
12. Casannes, J., “Sur la diffusion de la lumiére par les molé- 
cules des gaz transparents.”? Ann. Phys., Paris, (9) 
15:5-149 (1921). 
13. CHANDRASEKHAR, S., “‘On the Radiative Equilibrium of a 
Stellar Atmosphere.”’ XXI—Astrophys. J., 106:152-216 
(1947); XXII—Ibid., 107:48-72, 188-215 (1948). 
14, —— “The Transfer of Radiation in Stellar Atmospheres.” 
Bull. Amer. math. Soc., 53:641-711 (1947). 
15. —— Radiative Transfer. Oxford, Clarendon Press, 1950. 
(See Chaps. IX, X) 
16. —— and Exrsert, D., “Polarization of the Sunlit Sky.” 
Nature, 167:51-55 (1950). 
17. Cornu, A., “Sur application du photopolarimétre 4 la 
météorologie.” C. R. Ass. frang. Av. Sci., Session 4 
Limoges, pp. 267-270 (1890). 
18. —— “Observations relatives 4 la couronne visible actuelle- 
