1872.] 



On Extraordinary Refraction in Iceland Spar. 



443 



Fluid collected Ammonia in grains Source 



in minims. per gallon. 



150 1*9712 Erysipelas. 



120 '1791 Garden. 



55 6-8807 Drains. 



90 2-1000 Bed-room. 



420 2-9568 Stables. 



150 '0985 Victoria Park. 



XI. " Contributions to Terrestrial Magnetism. — No. XIII." By- 

 General Sir Edward Sabine, K.C.B., V.P.R.S. Received 

 June 19, 1872. 



(Abstract.) 



The author presents this paper as the companion of No. XI. of his Con- 

 tributions to Terrestrial Magnetism, which contained the Magnetic Survey 

 of the Southern Hemisphere from 40° south latitude to the extreme limit 

 towards the southern pole, as does the present memoir, No. XIII. of the 

 same series, the three magnetic elements from 40° north latitude to the 

 furthest attained limit towards the northern pole. In both papers the 

 mean epoch is the same, viz. 1842-5. Where it has been possible to do 

 so, corrections to this mean epoch have been obtained and applied to 

 earlier and later observations. 



The determinations are derived from observers of all countries, and 

 are arranged in zones, each of 5° of latitude, passing round the globe. The 

 Table thus formed contains between 3000 and 4000 stations at which the 

 magnetic elements have been determined. The observers are named, and 

 references are made to the sources from whence their observations are 

 taken. The paper is accompanied by maps of the resulting Isogonic, Iso- 

 clinal, and Isodynamic Lines, executed at the Hydrographic Office. 



XII. "On the Law of Extraordinary Refraction in Iceland Spar." 

 By G. G. Stokes, M.A., Sec. R.S. Received June 20, 1872. 



It is now some years since I carried out, in the case of Iceland spar, the 

 method of examination of the law of refraction which I described in my 

 report on Double Refraction, published in the Report of the British 

 Association for the year 1 862, p. 272. A prism, approximately right-angled 

 isosceles, was cut in such a direction as to admit of scrutiny, across the 

 two acute angles, in directions of the wave-normal within the crystal com- 

 prising respectively inclinations of 90° and 45° to the axis. The direc- 

 tions of the cut faces were referred by reflection to the cleavage-planes, and 

 thereby to the axis. The light observed was the bright D of a soda-flame. 



The result obtained was, that Huygens's construction gives the true law 



VOL. XX. 9 K 



