120 



Sir David Brewster on the Polarization 



Fig. 2. 



til ill 



M. Babinet. It consists of two plates of 

 rock crystal which give a system of rec- 

 tilineal polarized bands*, attached to an 

 analyzing plate of tourmaline. In very 

 feeble lights I used another instrument, 

 in which the analyzer was a Nicol's prism. 

 When the polariscope is directed to a 

 neutral point, the system of fringes is in- 

 terrupted, as shown in fig. 2 at N P. The 

 bands on one side of the interruption are 

 oppositely polarized to those on the other, 

 the central band in the one being blacky and in the other white. 



In order to measure the altitude of the neutral point thus ob- 

 served, I used a common quadrant with a plumbline, which gave 

 within half a degree the altitude of the centre of the neutral point. 

 In order to obtain the height of the neutral point above the sun, 

 or the antisolar point, it was necessary to have the sun's altitude, 

 or his depression beneath the horizon. Having asked Professor 

 Henderson for the easiest method of obtaining these with a 

 moderate degree of accuracy, he recommended to me to use 

 Margett's Longitude Tables, from which they were accordingly 

 taken. 



The following observations are a selection of some of the most 

 important out of a very great number recorded in my journal t« 

 In order not to crowd the page with figures, I have given only the 

 hour of observation, and the altitude of the neutral point above 

 the antisolar point, and occasionally the maximum polarization J. 



1841, May 12.— Barom. 30-1 in.; therm. 9 h p.M. 48°. 

 sky unusually clear. 



The 







R= maximum 



polarization. 



Height of neutral point 



Mean time. 



h m 



7 10 P.M. 





In zenith. 



In horizon. 



above the antisolar point. 





30°| 



o 



28 



20 33 



7 25 „ 









20 29 



7 35 „ 





30| 



29| 



21 13 



7 45 „ 





28^ 



28 



21 33 



7 55 „ 



The 



sun set. 





22 8 



8 5 „ 





. , 



, 



21 35 



8.15 „ 









20 41 



8 30 



The polarized bands faint. 



22 26 



.* See Edinburgh Transactions, 1819, vol. ix. p. 148, where the method 

 of producing rectilineal bands by crossing two plates of rock crystal was 

 first published. 



f The rest of these observations will, probably, be published in an early 

 volume of the Edinburgh Transactions. 



X See the section on the place of maximum polarization, and its intensity 

 or R. 



