POLARISED LIGHT.
During the last summer, while amusing myself with verifying a statement of Sir D. Brewster respecting the light of the rainbow, viz. that it is polarised in particular planes, I observed a phenomenon which startled me exceedingly, inasmuch as it was quite new to me at the time; and, notwithstanding subsequent inquiries, I cannot find that it has been observed by any other person. I found that the light of the blue sky is partially polarised. When analysed with a Nicols' prism, the contrast with the surrounding clouds is very remarkable; so much so, indeed, that clouds of extreme tenuity, which make no impression whatever on the un-assisted eye, are rendered plainly visible.
The most complete polarisation seems to take place near the horizon; and, when the sun is near the meridian, towards the west and east. The depth of colour appears to be immaterial, as far as I have been able to ascertain with an instrument but rudely constructed for the purpose. The light is polarised in planes passing through the eye of the observer, and arcs of great circles intersecting the sun's disc.
From the absence (so far as I am aware) of all mention of this remarkable fact in works on the subject, I am led to conclude that it is something new; should this, however, turn out otherwise, I shall be obliged by a reference to any author who explains the phenomenon. The greater intensity towards the horizon would point to successive refractions as the most probable theory.
H. C. K.