POLARIZATION OF SKYLIGHT 81 
much simpler. Their elevation above the horizon is 
measured by standard procedures (usually by a pen- 
dulum quadrant or a theodolite); for finding the neutral 
point any type of polariscope can be used. The most 
convenient type is the Savart polariscope and its modi- 
fications. The main part of the polariscope is Savart’s 
double plate (two plates of the same thickness cut 
under 45° from the optical axis of a quartz or any 
uniaxial crystal; one of them turned through a right 
angle from the other). If a polarized light passing 
through the double plate is observed through an ana- 
lyzer with the plane of polarization bisecting the angle 
between the principal planes of transmittance of the 
plate, parallel color fringes appear. They have a dark 
or bright central band, depending on whether the in- 
cident light is polarized at right angles or parallel to 
the plane of transmittance of the double plate. The 
fringes disappear if the incident light is unpolarized. 
The modifications of Savart’s polariscope differ with 
the type of analyzer used. In the original model a 
tourmaline plate was used. Its great disadvantage was 
a strong absorption resultmg in a dark green color of 
the field. By using a Nicol or similar prism this dis- 
advantage can be removed, but the field is then very 
small. Much larger fields and an extraordinary bright- 
ness of fringes can be reached in Voss’s modification [69] 
with a Wollaston prism as analyzer. By a suitable ad- 
justment of the thickness of the double plate and the 
Wollaston prism, the deviation of the ordinary and 
extraordinary rays emerging from the prism can be 
made exactly equal to the angular distance of the im- 
terference fringes. In this way the frmges in the ordi- 
nary and extraordinary system of rays coincide and 
their intensity is doubled. Because of its great lumi- 
nosity the Voss polariscope is very useful for measur- 
ing neutral points late after sunset. Its colorless field 
makes it particularly useful for measurements within a 
narrow spectral zone. The advantage of a colorless field 
can also be achieved by using a polaroid plate as ana- 
lyzer [45]. 
If the polariscope is set up with fringes parallel to 
the sun’s vertical in the vicinity of a neutral point, the 
dark central band above the point continues as a bright 
one below with an interruption in the middle in the 
exact position of the neutral poimt. The elevation of 
this point is then measured. The position of the inter- 
ruption in the fringes can also be determined photo- 
eraphically [6]. 
Distribution and Magnitude of the Polarization over 
the Sky 
As already mentioned, the degree of polarization of 
skylight reaches its maximum in the sun’s vertical, 90° 
from the sun. Mean values of a large number of meas- 
urements of the polarization at this point taken in 
different years and at different places, agree relatively 
well, showing a decrease of the degree of polarization 
with increasing elevation of the sun (Fig. 1). Measure- 
ment of polarization at the zenith was introduced by 
Jensen [81] and performed by several authors because 
of the simplicity of having a fixed position of the ob- 
served direction independent of the sun’s elevation. 
With the sun at the horizon, the zenith coincides with 
the pot of maximum polarization. With the sun above 
the horizon, the polarization at the zenith decreases 
rapidly as the point of maximum polarization descends 
AHLGRIMM 
TICHANOWSKI 
POLARIZATION 
(eo) 
NI 
o° 20 
SUN'S ELEVATION 
_ Me. 1—Polarization at the point of maximum polarization 
(in the sun’s vertical, 90° from the sun) for different sun’s 
elevations h,. Observed values (Dorno, Gockel, Tichanowski) 
compared with theoretical values (secondary scattering ac- 
cording to Ahlgrimm). 
40° 
1.0 
I 
r 
TICHANOWSKI 
Zz 
[o} 
5 GOCKEL 
N 0.5 
cc 
a 
=| 
oO 
a JENSEN 
DORNO 
fo} 
(ay 20° 40? 
SUN'S ELEVATION 
Fig. 2.—Polarization at zenith for different sun’s elevations 
hs. Theoretical values (I—Rayleigh’s theory of primary scat- 
tering, II—secondary scattering according to Ahlgrimm) com- 
pared with observed values (Tichanowski, Gockel, Jensen, 
Dorno). 
towards the horizon. The daily variation of the polari- 
zation at the zenith has thus the same character as 
that of maximum polarization, but with a much larger 
range of variation, as may be seen in Fig. 2. The meas- 
urement of polarization at the zenith, extended for 
negative sun’s elevations h,, gives an interesting result: 
the maximum polarization at the zenith is reached for 
