Reflexion of Light from Iceland Spar. 247 



Table II. 





w. 



tan sr. 



T 

 I. 









1 



53'- 5 



_._ .. . . 



03303 



- 



108-185° 





1 



50-8 



0-03225 



108-033 





1 



48 - 1 



0-03145 



108-105 





1 



52 5 



-03275 



108-259 





2 



0-3 



0-03501 



108 -151 





1 



55 3 



\J WOOOO 



1 AS -ASQ 





1 



57 5 



-03419 



108 -099 





1 



49- 



-03172 



108-101 





1 



50-4 



03212 



108-102 





2 



5-4 



'03648 



108 -024 





2 



3-2 



0-03585 



107 -875 





1 



51 -2 



0-03236 



108-142 





1 



56-8 



-03399 



107-986 



Means . . 



1 



54-9 



0- 03344 



108-088 



Taking from Table I the mean values of R' — R, r' — r, we have — 



_ sin (V— r) sin 85*886 „ ^-^^ 

 cos 2*r = . / \ ) , J. = -— -—^ = 0-997762. 

 sin (R — R) sin 91*492 



Thus vr = 1° 55', and we obtain for tbe values of the quantities 

 which, determine the nature of the polarised light, 



tan v = 0-03346, I = 108*088°. 



We have to find whether these two quantities are altered by 

 polishing. 



The subsidiary quantity p is given by — 



tan (r'-r) - tan 85*886 _ 



COS p = rhi ~r = tt— — = —0*3621. 



r tan (K- R) tan 91*492 



We are calculating p merely for the purpose of verification, and are 

 not using it to determine the nature of the polarised light. Take, 

 then, p to be the least positive angle which satisfies the last equation. 

 Thus p = 111° 14'. This is the mean value of p. To estimate the 

 error in determining p, take the 12th set of observations in Table I. 

 We have — 



tan 85*900 _ * 

 C0 ^ = ta^9T756 = -°' 42768 > 



whence p = 115° 19'. There is thus a difference of 4° in the extreme 

 value of p from the mean. The cause of this apparently large varia- 

 tion in the values of p will be considered later on. 



