SCIENTIFIC SUMMARY. 
187 
The explanation has generally been referred to reflection of the solar rays 
for the vertical, and to refraction joined to secondary reflections, for the 
horizontal polarization. The neutral points thus become spots where the 
vertical and horizontal actions have equal and opposite intensity. Hypo- 
theses regarding the method of reflection have been founded on the partial 
opacity of the air itself, on that of suspended solid particles, and on small 
vesicles of water. Hagenbach referred it to layers of air of unequal density. 
Tyndall considers it not due so much to simple reflection as to a peculiar 
phenomenon which attends on luminous gas. 
Wheatstone, by means of an apparatus which he termed a Polar Clock, 
showed that by pointing this to the pole the position of the plane of polar- 
ization would approximately give solar time. M. Becquerel set himself to 
determine with precision the relative positions of the sun and of the plane of 
polarization. 
The apparatus he devised for these researches consists of a Savart polari- 
scope mounted in a divided circle, and observed by means of a total reflection 
prism. It can be turned in any direction, azimuth and altitude circles giving 
independently the co-ordinates of the point observed with reference to the 
magnetic north. 
The observations consisted in determining on the same divided circle the 
position of the plane of polarization, and the shadow of the optical axis of 
the apparatus thrown by the sun itself. Cross wires were fixed to the end 
of the movable tube which threw an image on a plane surface attached to it. 
These were afterwards replaced by two needle-points which threw their 
shadow on a small screen regulated to move in the sun’s plane. The Savart 
polariscope was found more delicate than others depending on equality of 
tint in the presence of much diffused non-polarized light ; but instead of 
observing the greatest intensity of the fringes, their disappearance at an angle 
of 45° was noted, which proved more susceptible of accurate determination. 
The process of observing consisted in taking three double measurements 
of the solar plane, with the time. Then a series of observations of the plane 
of atmospheric polarization followed, and then again a repetition of the first 
measurements. These were plotted out with lines as Abscissse and the 
numbers found as Ordinates. The former proved the more accurate. 
Several of them are reproduced in the memoir. 
The results, stated briefly, are as follows: 1. The plane of polarization 
from any given point in the sky does not generally pass through the sun, but 
usually below it. At the zenith, the planes of the sun and of polarization 
coincide. At the pole their angle is small, and was hence unnoticed by 
Wheatstone. They increase towards the horizon. As a rule the polarized 
light coming from the sky behaves like a luminous ray starting from the 
neighbourhood of the sun and reflected towards the observer; but it contains 
rays polarized by refraction also, of variable intensity, in some cases sufficient 
to annul the opposite effect, and to produce ‘ neutral points.’ Indeed the sun 
is not to be regarded as the only source of atmospheric illumination, but also 
the air and the earth acting as reflectors. Each of these may displace the 
plane. 
As regards the action of terrestrial magnetism, it appears that the plane 
of polarization undergoes rotation in a direction always the same, direct if 
