the Light reflected by the Sky and by Plates of Glass. 135 



ville, N. H., in a valley at a height of 1500 feet, surrounded by 

 mountains 4000 feet high ; the other two from the top of the 

 Institute of Technology, Boston. It soon became evident that 

 the polarization depended on the solar distance of the point under 

 observation, and not on its altitude. This is well shown in 

 fig. 5, in which abscissa? give the solar distances, and ordinates 

 the polarization of the observed points. The sun's altitude 

 varied in these series from 0° to 68°. 



Before discussing these observations further, it seemed desi- 

 rable to determine the polarization of other parts of the atmo- 

 sphere not lying in the same vertical plane with the sun. More- 

 over, as the polarization of points at equal distances from the 

 sun should be compared, the polari meter was so mounted that 

 its principal axis would pass through the sun. The two gra- 

 duated circles would then give solar distances, and the angle 

 from the vertical plane through the sun instead of altitudes and 

 azimuths. The second of these angles will be called the meri- 

 dian-distance, and will be regarded as positive to the right, and 

 negative to the left of the sun. Of course, the direction of the 

 axis should continually change, so as to follow the sun ; but as 

 great accuracy in the determination of the angles was not needed, 

 it was found sufficient to readjust it every few minutes. Another 

 advantage of this arrangement was, that the line of junction, 

 being turned parallel to the axis, would always lie in the plane 

 passing through the sun, and hence be parallel to the plane of 

 polarization. 



Five series of observations were made in this way, all tending 

 to show that the polarization was independent of the distance 

 of the point to the right or left of the plane passing through the 

 sun. The results are given in Table IV., in which the first co- 

 lumn gives the number of the series, the second the sun's alti- 

 tude, the third its distance from the points observed, the fourth 

 the number of the latter, and the fifth their mean polarization. 



Table IV. — Polarization of Points equidistant from the Sun. 



Series. 



Altitude. 



Distance. 



No. 



Polarization. 









o 







12 



5 



90 



9 



767 



13 



20 



60 



6 



42-9 



14 



48 



90 



8 



65-4 





45 



60 



10 



417 





42 



120 



2 



424 





40 



30 



8 



107 



15 



40 



90 



10 



60-4 





41 



120 



9 



409 





43 



60 



10 



34-2 



16 



-5 



90 



6 



77'5 



