1^2 Sir David Brewster on the Polarization 



degrees of rotation, R, therefore, of the planes of polarization 

 produced by the refraction of the plate EL at different inclina- 

 tions to the axis of the tube may be calculated from the for- 

 mulae 



R = </>-45°, 

 and 



cot<£=cos 2 . (i—i f ), 



i being the angle of incidence, and i' the angle of refraction. The 

 values of R (the measures of the degrees of polarization) being 

 laid down on the tube E F, we have a polarimeter which gives 

 us direct measures of the polarization of the atmosphere, or any 

 other kind of polarization, when it does not exceed the maxi- 

 mum polarization produced by the refraction of one plate. In 

 this instrument the zero of the scale is at F, and B is the index. 

 In place of one plate, E L, we may use two, three, or more 

 plates, and thus obtain a measure of all degrees of polariza- 

 tion. If n is the number of plates, the value of R will be as 

 o Hows: — 



R = <£-45°, 



cot $= cos w (i— i'). 



In the instrument which I constructed, the radius A C was 1*13 

 inch, and the scale on F B was laid down from the following 

 Table, the index of refraction of the plate EL being 1*51. 



Angles of incidence i. Angles of refraction i'. Rotation R. Length of tangent, 



o / o i / in inches. 







30 19 20 1 0-655 



40 34 25 31 2 0-968 



47 46 29 22 3 1-244 



53 17 32 4 4 1-515 



57 39 34 1 5 1-784 



61 25 35 3 6 2074 



64 40 36 46 7 2-386 



67 29 37 43 8 2726 



70 38 29 9 3-104 



72 19 39 7 10 3-544 



By setting off the tangent 0*655 inch from the zero at F, we 

 obtain the place of 1° of rotation, and so on with the rest. The 

 polariscope P is placed at the end F H of the tube. 



Having observed the maximum polarization of the atmosphere 

 by the polarimeter, and its place, we take its altitude A, and by 

 means of the sun's altitude A', observed or computed, we obtain 

 the distance D of the place of maximum polarization from the sun, 



D = 180°-A + A 7 . 



