hy transmission through Fluids. 423 



a circularly polarizing medium between the analyser and po- 

 larizer, the plane of polarization will be removed to the right- 

 hand or to the left. In consequence of such change in the 

 plane of polarization, X will become lighter and O darker. 

 When the rotation of the analyser proceeds in the direction of 

 the numbers in the ordinary mode of graduation, the rotation 

 is termed r/^^^-handed, when in a contrary direction left- 

 handed. 



Supposing the rotation produced be 30° to the right, then 

 the analyser must be also shifted 30° to the right-hand, in 

 order to obscure X; proceeding onwards, it will be observed 

 that X becomes bright at 120°, being a further revolution of 

 90° ; again becomes obscured at 210°, or a revolution of 180°, 

 and again bright at 24'0°. On registering, then, the degrees at 

 which O and X are respectively polarized, that is, most ob- 

 scured, we have the following table : — 



X O X o 



30° 120° 210° 300° 



the degrees being observed on a right-handed graduated circle, 

 and corresponding to 



X O X o 



330° 240° 150° 60° 



on a circle graduated in a left-handed direction. 



Had we moved the analyser in a left-handed direction, we 

 might therefore have registered our results as follows: — 

 O X O X 



60° 150° 240° 330° 



by which it will be seen that it would be impossible, from the 

 appearance presented at any given position, or the direction 

 in which the analyser was revolved, alone considered, to say 

 whether the deviation was 30° to the right or 150° to the left; 

 and this would of course be equally true of any other amount 

 of deviation. 



To add to the difficulty, the deviation is in direct propor- 

 tion to the thickness and nature of the circularly polarizing 

 medium. If you find, for example, a deviation of 10° pro- 

 duced by a medium of 1 inch in depth, a depth of 10 inches 

 would produce a deviation of 100°; so that by merely vary- 

 ing the thickness or depth of the medium, you may have the 

 position of greatest obscuration situated in any part of the 

 circle, and if the thickness happened to be such as that such 

 position coincided with 180° or 0°, you might suppose that 

 the medium was not possessed of circular polarization at all; 

 the importance, therefore, of examining different depths or 

 thicknesses will be apparent; and the depth of each medium 



