222 



SIR DAVID BREWSTER 



+ 



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 I - 



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Fig. 3. 



polarisation was overcome by the advancing negative polarisation. The negative 

 polarisation was then immediately below the ascending neutral point ; but at a 



certain distance (a few degrees below the neu- 

 tral point), the negative polarisation was com- 

 pensated by the excess of positive polarisation 

 close to the horizon, and the beautiful pheno- 

 menon was seen of two neutral points, a primary 

 and a secondary, separated by bands of negative 

 polarisation, as shown in the annexed figure. 



1841, June 10, 6"^ 40°'.— The neutral point 

 a little above the horizon, with vertical or + 

 polarisation 07i both sides of it. The new verti- 

 cal polarisation had more than compensated the 

 horizontal, or negative polarisation, and left a 

 balance of positive polarisation, which soon dis- 

 appeared when the rising horizontal polarisation 

 overpowered it. 



1842, Feb. 22. — Both on this day and on the 13th, the neutral point was 

 above the horizon, though not visible, being eclipsed or masked by the cause 

 which produces the secondary neutral point. Over a space of 3i° above the 

 sea, the positive bands almost wholly disappear before the negative bands are 

 perceptible, and the neutral point is 5° high when the secondary neutral point is 

 distinct in the sea horizon. 



Although I have observed the secondary neutral point more than twenty-two 

 times, it has generally appeared under slightly different forms, varying with the 

 intensity of the new polarising cause which produces it, and with the point of 

 the horizon where the neutral point rises 



It is unnecessary to describe these different forms, — I shall mention only an 

 observation made on the 21st April 1849, under very favourable circumstances. At 

 6^ 22", when the primary neutral point was about 15° high, the secondary neutral 

 point was 2° 50' high, the negative bands covering a space of 8^ or 9" between 

 them, the positive bands being above the sea-line. A fog prevailed to some 

 extent, and above the sea-line there was the dark purplish belt previously men- 

 tioned, over which the positive bands were stronger than on the part of the sky 



above it. 



Observations on Babinet's Neutral Point. 



In the year 1840, when M. Babtnet had occasion to visit the sea coast, he 

 proposed to observe if the neutral point of Arago varied in its height as the sun 

 rose or set, and to observe it also when the sun was beneath the horizon ; but he 

 was allured from these observations by a circumstance which he had never even 

 suspected, namely, the existence of a second neutral point above the setting sun, 



