in the Neighbourhood of the Polarizing Angle. 15 



quantity, but it is not without interest from a theoretical 

 point of view. The fact that k can be positive as well as 

 negative implies of course its possible evanescence. It is, I 

 think, safe to say that some samples of water-surfaces polarize 

 light to perfection. 



It will now be desirable to give a specimen of actual obser- 

 vations. The one selected, principally on account of its 

 completeness in respect to the positions of the quarter-wave 

 mica, is dated April 1, 1891, and relates to a surface of tap- 

 water, freshly drawn, and skimmed with the aid of the brass 

 hoop already described. The operation of skimming was 

 repeated after each readjustment of the mica. In the first 

 column the direction of the arrow indicates the position of the 

 mica. The second gives the readings of J3, the third the indi- 

 vidual readings of a, the minutes only being entered*. It 



Mica. 



Analyser, 

 (3. 



Polarizer. 



Separate Headings of a. 



(minutes). 



Means. 



Difference. 



J 



o 



+30 

 -30 



+30 



-30 



+30 

 -30 



+30 



-30 



31, 32, 32, 31 



28, 29, 29, 29 



28, 31, 29, 30 



32, 31, 32, 32 



33, 33, 33, 33 



30, 32, 32, 32 



28, 29, 29, 29 



31, 32, 31, 29 



183 314 



183 28| 



183 29| 

 183 31| 



183 33 

 183 31^ 



183 28f 

 183 30| 



+2| 



-2i 



_9 



will be understood that the readings for &= ±30° were taken 

 alternately ; the first reading under j being 31', the second 

 28', the third 32', and so on. The fourth column gives the 

 means, and the fifth the difference of these means, which 

 represents ellipticity. The second and fourth differences, 

 corresponding to positions of the mica differing by a right 

 angle from those of the first and third, must have their signs 

 reversed before combination for a final mean difference. We 



get 



i(2f + 2i + li + 2)=+2i/. 



After the last set the hoop was lifted, so as to allow the return 



* In almost all the observations the settings were made by myself, and 

 the reading's of a, at the vernier by Mr. Gordon. Without two observers 

 the difficulties would have been much increased. 



