98 Whe( I< /• - Expt rvrrn ntal In vestigation on the RejU otion 



two values of the displacement of the compensator correspond- 

 ing to a change of phase of 300°. The results, in terms of 

 turns of the compensator screw, follow : 



Polarizer set to 



3] -58* 



201-58° 



111-58° 



291 -58* 



Compensator (left) . . 

 (right). 



8-988 



8-1171 



8-967 

 8-982 



8-982 

 81)74 



8962 

 8-979 



(av.)-.- 8-980 8-975 8-978 8-970 8-9756 ±0-0021 



Compensator left or right means that the first fringe to the 

 left or right of the central one was under the cross wires. On 

 two occasions in the course of the investigation it became nec- 

 essary to take the compensator to pieces, once to clean it and 

 the second time to replace the cross wires. On each occasion it 

 was re-calibrated and both times with the same result within 

 the probable error of the determinations. The constant of the 

 compensator, i. e., the factor by which its readings must be 

 multiplied in order to reduce them to degrees, deduced from 

 the above readings is 40-11. The position of the central 

 fringe (zero point of the compensator) was found to vary 

 slightly with temperature but otherwise was constant. The 

 maximum variation observed was 0"035 turns. The scale on 

 the compensator is so placed that its direct readings yield the 

 supplement of the angle of phase difference rather than the 

 angle itself. 



The observations of the phase difference and the angle of 

 restored polarization were made in the following manner in 

 every case : (1) With both the telescope and collimator arms 

 level and the surface-cleaning apparatus lowered out of the 

 way, the zero point of the compensator was determined as the 

 mean of at least twelve settings taken coming up from both 

 directions. (2) With the two arms set to give an angle of 

 incidence <f> = 60°, the mirror was brought into position, 

 adjusted and cleaned in the manner which has been described. 

 (3) With the polarizer set to give light vibrating in a plane 

 inclined at an angle of 45° to the plane of incidence, the dis- 

 placed position of the central fringe was observed (again as the 

 mean of at least twelve settings from each direction), and the 

 position of the analyzer making the fringes blackest was 

 determined. For this last at least twenty-fo.ur settings were 

 made in each case. (4) With the polarizer set to give light 

 vibrating in the other 45° plane, the compensator and analyzer 

 positions were again determined and with the same number of 

 readings for each. (5) Finally, with the arms brought back 

 into line, the zero point was again determined as in (1), so as 

 to be able to take account of any temperature change in the 

 compensator. The reflecting surface was renewed, of course, 



