ASTRONOMY AND METEOROLOGY. 321 



of each view, unbiased by any theory, leaving to those best able 

 to judge to determine whetlier either explains all the tacts ob- 

 served. The absence of a lunar atmosphere is so generally ad- 

 mitted, that its existence is suggested only with reluctance, and 

 merely as the most natural explanation of the observations. 



Note on the Supposed Polarization of tlie Corona. — The form of 

 the instrument used has been, in one or two cases, misunderstood. 

 It consisted of a sheet-iron tube, closed at one end with a plate 

 of quartz, and at the other with a prism of Rochon. The latter 

 has the property of giving two images of an}^ object seen through 

 it, separated by an angle of nearly o°. Looking through the tube, 

 we therefore see two images of the quartz touching, but not over- 

 lapping. When the light is polarized, these images assume com- 

 plementaiy tints, which vary with the plane of polarization and 

 the thickness of the quartz. 



The corona appeared white ; but the sky surrounding it was col- 

 ored in one image blue, in the other yellow. The conclusion to 

 be drawn from this is, that the light of the corona is unpolarized, 

 or, more strictly, that the amount of polarized light, if any, is too 

 slight to be perceptible with this instrument. Its delicacy, although 

 not equal to Savart's polariscope, is very great, giving colored 

 images with paper, wood, and other bodies which reflect a small 

 amount of light specularly. The day before the eclipse, it showed, 

 in a very marked manner, the polarization of the wet pavements 

 and roofs. To measure its sensitiveness, I viewed the li<rht re- 

 fleeted by a piece of plate glass, at dift'erent angles of incidence, 

 and found that the color ceased to be visible when this angle was 

 about 10°, which, allowing for the reflection from the second face, 

 would give about one part of polarized to 24 of natural light. 



Observers heretofore have generally attached their polariscope 

 to a telescope, and thus introduced a source of error, avoided in 

 my instrument. For light passing through tfte object-glass and 

 field-lens would be polarized by refraction, before reaching the 

 polariscope, by the obliquit}' of the incidence, caused both by the 

 curvature of the surfaces and the fact that the edge of the field 

 of view receives its light not parallel to the axis. The plane of 

 polarization would be perpendicular to a plane falling through 

 the axis of the instrument. Now, if any part of the corona was 

 brought into the centre of the field of view, the adjoining portions 

 would appear polarized in planes parallel to the edge of the field, 

 or passing through the .sun's centre. In sweeping around the 

 sun's edge, the plane of polarization would continuall}' change, 

 as the corona passed through dift'erent parts of the field, and the 

 comparative darkness of the moon's disc and the exterior sky pre- 

 vent the polarization of the other portions of the field from being 

 visible. The degree of polarization by refraction would be very 

 slight, and, perhai)s, impcrceiDtible ; but the agreement of obser- 

 vation wiih this hypothesis is certainly a curious coincidence. 



The stronge.-t argument against the polarization of the corona 

 is furnished by the spectroscope, the presence of bright lines and 

 absence of dark on(;s, as observed bv Prof. Young, denoting in- 

 candescence, — a view strengthened bv the consideration that 



