242 Prof. L. P. Whealer on the Reflexion of 



were made in the instrument shop o£ the Sheffield Scientific 

 School. The divided circle of the spectrometer table could 

 be read to 20" and those of the nicols to 6' of arc. The 

 source of light used was that obtained from the sodium flame 

 of a small blast-lamp fed with oxygen. 



Measurements. 



The preliminary adjustments of the apparatus, i. e. making 

 the axes of collimator and telescope to intersect the axis 

 of the spectrometer table orthogonally, and the adjustment of 

 the axes of the nicols to parallelism with tho?e of the colli- 

 mator and telescope, were made in the usual manner. The 

 positions of the polarizer giving light polarized in the plane 

 of incidence was determined by reflexion from waier at the 

 polarizing angle. The water surface was cleaned in the same 

 manner as described above for the mercury. This plane 

 was determined several times in the course of the investi- 

 gation and with several minor modifications of method. 

 Thus, sometimes the extinction position of the analyser was 

 determined with the reflexion taking place at the polarizing 

 angle, and then that of the polarizer by getting its "crossed" 

 position with reference to the analyser when the two were 

 put in line ; or, sometimes the extinction position of the 

 polarizer for the reflexion at the polarizing angle was deter- 

 mined directly, the beam of light being sent through the 

 apparatus in the reverse direction ; another modification was 

 the use of glass of known index in place of water. None of 

 these various determinations yielded a position of the polarizer 

 differing from that of any other determination by an amount 

 as large as the probable error of the readings. The result of all 

 these determinations gives a value of 1T)6 0, 58 for the position 

 of the polarizer. The probable error is less than o, 003. 



The calibration of the compensator was made with the 

 polarizer in all four of the positions yielding light vibrating 

 in a plane making an angle of 45° with the plane of incidence. 

 For each position the reading for the centre fringe and the 

 first fringe on either side of the centre was determined as the 

 mean of fifteen settings. Thus each position of the polarizer 

 yields two values of the displacement of the compensator 

 corresponding to a change of phase of 360°. The results, in 

 terms of turns of the compensator screw, follow : — 



Polarizer set to 



. 20° "58 



201°-58 



lll°-58 



290° -58 



Compensator (left) .. 

 (right) .. 



. 8-988 

 . 8971 



8-967 

 8-982 



8-982 

 8-974 



8-962 

 8-979 



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



