114 E. C. Pickering—Measurements of the 
Evidently we may conclude from these colors that the true 
color of the sky particles is blue, a view quite in accordance 
with the observations of Prof. Cooke with the spectroscope, 
and Prof. Tyndall on aqueous vapor in a state of formation. . 
Observations were next made to test the results formed above 
for the light reflected and transmitted by several parallel sur- 
ment of the angle of incidence. line of junction of the 
two images was then rendered vertical, and the polarization 
measured in the usual way. The polarization of the sky, 1 
tical. At any other angle, a, the vertical and horizontal com- 
onents were cos?a and sin?a, and hence were equivalent to 4 
eam polarized vertically by an amount cos2a. If now any 
object was inserted between the two telescopes polarizing the 
light horizontally, p, the bands would disappear only when p= 
cos2a. Measuring the four positions of disappearance, and tak- 
ing their mean, gave an accurate measure of the polarization by 
a table of natural cosines, as with the polarimeter described 
above. Another way of expressing the effect of this instru- 
ment is to say that the bands disappear when the Nicol 1s 80 
turned that the plane of polarization shall be brought by the 
object under examination to an angle of 45°. The method of 
measuring the polarization of the reflected ray is now obvious. 
The pieces of glass are placed vertically on the center plate 
between the two telescopes, the latter set at an angle of 22, and 
_ the glass turned until the light is reflected from its surface, 5° 
as to render thé field bright. The Nicol is then turned until 
the bands disappear, and its position recorded. The angle be- 
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