22 



Mr. E. A. Houstoim on the Efect of a 



between the two positions o£ B, which could be read on the 

 scale, was the angle between the directions of A and C. This 

 angle was 41° 47'. 



Before taking readings of the phase-difference, the table 

 was rotated and the platform adjusted, so that the light 

 from the collimator was reflected on a long side of the first 

 rhomb, making the round opening of the collimator A 

 coincide with the cross-wires of the telescope C. The 

 position of the table was then noted. The same was then 

 done for the second rhomb. If the readings of the scale 

 were in these cases a and ft, and the angle between A and 



C y, then if the reading on the scale were ^-J- , the bisector 



of angle 6 would bisect angle y. Consequently, when the 

 rhombs are in the correct position, the scale-reading must be 



^ -• Tne table was tnen placed correctly and the 



platform displaced until the light from the collimator fell on 

 the most favourable part of the end surfaces. 



To find the angle of incidence <£ we proceed as follows. 



In the position of symmetry i= - 



sin i = n sin r = n sin (<£ — 54° 40'). 



And 6 = 7r— (j3 — «) — 2(54° 40'): hence $ can be calculated. 

 Two series of observations are given in the table. A, A , 

 A ; , and c£ have the same meaning as before. 



Observations on Fresnel's Bhoinbs. 



I. 







II. 







<p. 



A. 



A - 



o 



A -A. 



A -A'. 

 o 



<p. 



A. 



V 



A -A. 



A -A' 



o 



o 



o 



o 



o 



o 



o 



o 



48-4 



42-0 



45-8 



3-8 



44 



51-4 



430 



4(5-6 



3-6 



37 



51-9 



43-2 



46-4 



3-2 



3-5 



52-8 



42-8 



46-2 



3-4 



3-4 



54-8 



42-1 



45-4 



3-3 



3-0 



54-8 



424 



45-4 



3-3 



3-1 



56-1 



42-5 



44-6 



2-1 



2-8 



56 - 5 



41-4 



44-2 



2-8 



2-8 



59-1 



39-3 



42-0 



2-7 



2-4 



58-4 



40-3 



42-6 



2-3 



2-6 



59-9 



38-5 



41-2 



27 



2-4 



596 



39-3 



41-4 



2-1 



2-4 



62-9 



35-8 



38-0 



2-2 



2-0 



62-8 



35-5 



38-1 



26 



24 



: 0-035. 



=0-036. 



The accuracy does not seem to be very much greater than 

 in the former case, when the light was only once totally 

 reflected, probably because here the light was not always 

 totally reflected on the same parts of the surface, and the 

 thickness of the surface- film will vary from place to place. 



