ON THE POLARISATION OF THE ATMOSPHERE. 



213 



N 



s e 



? „lf,„,v,, .^i--l|l>,-'Ei'.-S'i'? .11- .? 



i I 



Observations on Akago's Neutral Point. 



In observing the different neutral points, I employed chiefly Savart's band 

 polariscope, which was kindly presented to me by M. Babinet. It consists of 

 two plates of rock crystal, which give a system of rectilineal polarised bands* 

 attached to an analysing plate of tourmaline. In very 

 feeble lights, I used another instrument, in which the 

 analyser was a Nicol's prism. When the polariscope 

 is directed to a neutral point, the system of fringes is 

 interrupted, as shown in Fig. 2 at N. P. The bands 

 on one side of the interruption are oppositely polarised 

 to those on the other, the central band in the one being 

 black, and in the other white. 



In order to measure the altitude of the neutral 

 point thus observed, I used a common quadrant with a 

 plumb line, which gave, within half a degree, the alti- 

 tude of the centre of the neutral spot. 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. 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 polarisa- 

 tion.f 



1 1 I 



I i 



Fia;. 2. 



1841, May 12.— Barom. 30-1 ; Therm. 9"^ p.m. 48°. The sky unusually clear 



Mean Time. 





R=Maximum Polarisation. 

 In Zenith. In Horizon 



7h IQm 



P.M. 





3or 



281° 



7 25 



„ 







... 



7 35 



!J 





30i 



291- 



7 45 



5J 





28i- 



28^ 



7 55 



„ The 



sun 



set. 



■ ■ ■ 



8 5 



?> 





• • • 



. • • 



8 15 



>5 









8 30 



/The 



polarized ) 





" ( bands 



faint. J 



. . . 



Height of Neutral Point ahove 

 the Antisolar Point. 



20° 



33 



20 



29 



21 



13 



21 



33 



22 



8 



21 



35 



20 



41 



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 See the section on the place of maximum polarisation, and its intensity or R., p. 245. 



