Reflexion of Light from Iceland, Spar. 251 



In order to test the truth of the results we have arrived at, a 

 Nicol's prism, the azimuth of which could be read off by a divided 

 circle to 3', was mounted on the side of the table opposite to that on 

 which the elliptic analyser was, in such a position that, when the angle 

 of incidence was the same as before, the light was reflected along the 

 tube containing the Nicol. It was found that the light reflected 

 from the surface could be reduced to a minimum but could never be 

 completely eclipsed. The minimum was very small but quite per- 

 ceptible. This might be due to the fact that the light was not homo- 

 geneous. But further experiments showed that, even with orange 

 and ruby glasses there was a perceptible minimum. Next the inci- 

 dence was increased and diminished in succession by 5°, and the 

 same result obtained. We, therefore, conclude as before, that the 

 light was slightly elliptically polarised. 



VIII. Discussion of the Determination of the Retardation of the Quarter 



Wave Plate. 



We have now to inquire into the discrepancy of the values of />, 

 which have been found before and after polishing, viz. : — p =111° 14' 

 and P = 116° 37'. 



Now, cos p is the ratio of the tangents of two angles, one of which 

 is very nearly 90°. Consequently, a very small error in this last 

 angle will produce a considerable change in the value of cos p. We 

 therefore come to this conclusion, that it is not a good plan to deter- 

 mine the instrumental constant p by reflexion from Iceland spar, 

 because the light reflected is so slightly elliptically polarised. To 

 determine more accurately the value of p, a steel plate was substi- 

 tuted for the crystal. At first, white light was used as before, but 

 the image was found to be strongly coloured. The difference in 

 readings for blue and red rays was several degrees. After trying a 

 spectroscope and coloured glasses, a ruby glass was selected as the 

 best. Then the observations recorded in Table Y were made. 



Table V. — Observations with a Steel Plate. Mean Temp. 17° C. 



r. 





R'. 



R. 



20 -468° 



74-705° 



89-296° 



5 -297° 



20-515 



74-661 



89 -082 



4-885 



20-529 



74 -334 



88-917 



5-097 



20 -495 



74-117 



88 -978 



5-286 



20 -692 



74-685 



89 -160 



5 305 



20-277 



74-135 



89 -034 



5-190 



20-520 



74 -282 



89-214 



5-635 



20-527 



74 -767 



89 235 



5-400 



