246 



Sir J. Conroy on 



[Jan. 9, 



plane of incidence probably varies more for each degree when the 

 light is incident at an angle greater than that of the principal inci- 

 dence, than when it falls on the surface at a less angle, the mean of 

 these two sets of determinations can only be considered as an approxi- 

 mation to the truth, especially when, as in this instance, the difference 

 between the two values is a considerable one. 



Mean value, from eight observations, four with the P. S. of the 

 polarising nicol inclined to the right and four with it to the left of 

 the plane of incidence, of the principal incidence and principal azi- 

 muth, for red light, with Quarter Undulation Plate 1. 





'Plate 



at A. . 



o 



80 



13 .. 



o 



.. 35 



52 







B . . 



70 



45 .. 



.. 35 



57 





.Mean 



value . 



75 



29 .. 



35 



54 





'Plate 



at A. . 



76 



46 .. 



.. 37 



13 







B. . 



66 



46 .. 



.. 35 



50 





.Mean 



value . 



71 



46 .. 



.. 36 



31 





'Plate 



at A. . 



76 



10 .. 



.. 37 



48 



Gold in carbon bisulphide. < 



" 



B.. 



62 



44 .. 



.. 37 



40 





I Mean 



value . 



69 



27 .. 



.. 37 



44 



The principal incidence and principal azimuth for gold in air, with 

 red light, were determined with six other quarter undulation plates 

 with the following results ; the numbers for Plate 1 being the mean of 

 eight observations, whilst those of the remainder are the mean of two 

 only, made with the polarizing nicol on either side of the plane of 

 incidence : — 



Plate 1. Plate 2. Plate 3. Plate 4. 



_ r Plate at A. . . 

 Principal i -g 



incidence ] " ' " " 

 LMean value. . 



Plate at A. . 

 , B.. 



Principal J 



azimuth | __ ' 



LMean value . . 



80 13 

 70 45 

 75 29 



35 52 

 35 57 

 35 54 



78 45 

 70 12 

 74 28 



36 30 



35 40 



36 05 



80 01 

 69 41 

 74 51 



36 12 

 36 02 

 36 07 



79 57 

 70 46 

 75 21 



36 22 



35 

 35 



10 



46 



Principal J 

 incidence 



Principal J 

 azimuth | 





Plate 5. 



Plate 6. 



Plate 7. 



Plate at A. . . . 



79 57 



77 43 



o / 



77 50 



„ B.... 



69 56 



73 26 



73 23 



Mean value . . . 



74 56 



75 34 



75 36 



Plate at A 



36 



34 55 



34 52 



„ B 



35 22 



35 10 



34 42 



Mean value . . . 



35 41 



35 02 



34 47 



