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XIII. " Observations made with the Polariscope during the 

 'Fox' Arctic Expedition." By DAVID WALKER, M.D., 

 Surgeon to the Expedition, in a Report transmitted by 

 Sir LEOPOLD McCLiNTOCK. Communicated by Professor 

 STOKES, Sec. U.S. Received March 7, 1860. 



The observations made with the polariscope*, with one exception, 

 were confined to solar halos and parhelia. Several times I tried the 

 instrument when lunar halos appeared, but the light was so faint that 

 only once was I able to make an observation. The direction of the 

 polarization was the same in all the cases observed, namely, in a plane 

 parallel to a line joining the part looked at with the centre of the 

 sun or moon. In several instances the instrument was turned round so 

 as to find the plane of maximum polarization, but it was always found 

 greatest in the parcel or perpendicular plane : when the plane was 

 oblique, no perceptible polarization at all was perceived. The light 

 was never completely polarized, the greatest amount not being more 

 than half; such occurred on 21st of April, and 5th and 6th of May 

 1859. All the halos observed had a diameter of about 45: there 

 were some seen of diameter 90, but the polariscope was not at hand 

 at the time. Almost always the halos round the moon or sun were 

 more or less prismatic, red internal. The observation on October 

 10th, 1857, was not on a halo, but the cloud which surrounded the 

 moon at a distance of about 1^ had circular and prismatic edges ; 

 light from these edges was slightly polarized, but not of the same 

 image as in all the other instances. 



Moon. 



October 7th, 1857. Polariscope applied to a halo round the moon, 

 diameter about 45 ; slight polarization ; arrow and brighter image 



* The polariscope employed consisted merely of a double-image prism of 

 quartz, formed of two quartz prisms cut in the usual manner and cemented toge- 

 ther, which was fixed at the eye-end of a tube about 18 inches long, provided at 

 its other end with a rectangular aperture having its edges parallel and perpendi- 

 cular to the planes of polarization of the two images, and of such breadth that the 

 two images just touched each other along one edge. The plane of polarization of 

 that image which was polarized in a plane passing through the axis of the instru- 

 ment, was marked by an arrow at the eye-end, lying in that plane, and placed on 

 the same side of the axis as the image in question. 



