188 Sir J. Conroy. On the Polarisation of [Feb. 4, 



Seeies II. 



Obs. at 0°, 10°, &c 51 40" + 5 cos - 3 11 cos 20 + 4 cos 30 - 17 cos 40. 



„ 5°, 15°, &c 51 32 + 7 cos - 3 12 cos 20 - 7 cos 30 + 10 cos 40. 



Seeies III. 



„ 1°, 11°, &c 52 05 - 11 cos - 3 16 cos 20-5 cos 30 + 13 cos 40. 



Mean (D) 52 02 - 5 cos - 3 14 cos 20 - 1 cos 30 + 13 cos 40. 



Iceland Spar in Tetrachloride of Carbon. Azimuths. 

 Seeies I. 



Obs. at 1°, 11°, &c 39-25 03 sin + 11 16 sin 20 + 4 28 sin 30 + 11 sin 40. 



-2° 40', 7° 20', &c. 50-23 31 sin + 10 11 sin 20 + 4 sin 30 - 44 sin 40. 



Seeies II. 



„ 0°, 10°, &c 56 - 23 26 sin + 10 17 sin 20 + 4 18 sin 30 - 49 sin 40. 



„ 5°, 15°, &c 28-23 07 sin 0+ 9 55 sin 20 + 4 23 sin 30 -12 sin 40. 



Mean (E) 43 - 23 47 sin + 10 25 sin 20 + 4 17 sin 30 - 24 sin 40. 



Iceland Spar in Tetrachloride of Carbon. Polarising Angles. 



Seeies I. 



Obs. at 1°, 11°, &c 54 05 - 17 cos - 9 40 cos 20 + 52 cos 30 + 1 18 cos 40. 



„ - 2° 40', 7° 20', &c . 53 01 + 4 cos — 8 45 cos 20 + 36 cos 30 + 59 cos 40. 



Seeies II. 



„ 0°, 10°, &c 52 50 + 12 cos - 8 41 cos 20 + 12 cos 30 + 1 23 cos 40. 



„ 5°, 15°, &c 52 41 - 3 cos - 8 29 cos 20 + 7 cos 30 + 1 10 cos 40. 



Mean (F) 53 09 - 1 cos - 8 54 cos 20 + 27 cos 30 + 1 12 cos 40. 



Artificial Surface of Iceland Spar in Water. Azimuths. 



(a.) .... 28' - 3° 52' sin - 9' cos + 5° 11' sin 20 + 11' cos 20 + 33' sin 30 + 7' cos 30 



-21' sin 40 — 4 cos 40. 



Artificial Surface of Iceland Spar in Water. Polarising Angles. 



(H.) 48° 53' - 1' sin - V cos 9 + 4' sin 20 - 2° 09' cos 20 + 4' sin 30 - 8' cos 30 



4 1 cos 40. 



Brewster, in his paper in the " Phil. Trans." for 1819, p. 158, says, 

 " in any given surface when A and A" are the maximum and minimum 

 polarising angles, viz., in the azimuths of 0° and 90°, the polarising 

 angle A' at any intermediate aximuth «, may be found by the formula 

 A' = A + sin 2 «(A"-A)." 



This expression is the same as that given by the harmonic reduc- 

 tion of the observations set forth in the preceding pages, if we 

 assume that the smaller terms are due to errors of observation, as in 

 that case the expression for the polarising angle in air (B) becomes 

 58° 17-1° 15'cos20. 



