256 Mr. C. Spurge. On the Effect of Polish on the 



arrangement, therefore, the Nicol moves in azimuth, carrying the 

 quarter wave plate with it, while the quarter wave plate has likewise 

 an independent motion in azimuth. 



After taking 700 readings I became convinced that the accuracy of 

 this mode of using the analyser was less than previously. A sub- 

 sequent investigation of the theory, made by tracing the surface locus 

 of a point whose coordinates represented the intensity of the trans- 

 mitted light, and the azimuths of the Nicol and quarter wave plate, 

 seemed to confirm this conclusion. 



It is well known that in the case of some bodies the application of 

 heat considerably influences the mode in which they reflect light. 

 Thus, in the case of flint glass, Sir David Brewster produced as great 

 an alteration as 9° in the polarising angle by varying the temperature.* 

 To test the alteration in the case of Iceland spar, a small tray filled 

 with ice was placed on the top of the crystal, all the rest of the appa- 

 ratus being carefully protected, especially the woodwork. I found 

 that, leaving the quarter wave plate and Nicol untouched, no per- 

 ceptible effect was produced by lowering the temperature of the 

 crystal 8°, and also that no difference could be detected when observa- 

 tions were taken. I conclude that in the case of Iceland spar for 

 moderate ranges the effect of temperature is insensible. 



XII. New Series of Observations. 



Since the change in the mode of using the elliptic analyser de- 

 scribed in the last section had diminished the accuracy of the obser- 

 vations, it was considered best to revert to the original arrangement 

 of the parts of the analyser. The instrument was therefore reversed. 

 The screw which fixed the stem of the analyser was unloosened, and 

 the disk rotated through two right angles about the stem as axis. The 

 tube was set to point in the same direction as before by means of the 

 diaphragms and screens. The quarter wave plate was unscrewed 

 from the collar and the Nicol from the disk. The quarter wave 

 plate was now connected with the disk and the Nicol with the collar. 

 Thus the arrangement of the parts of the analyser was now exactly 

 like that described in Section II. 



Since the quarter wave plate has been unscrewed, we must no 

 longer expect the values of I to be the same as in Table IV. There 

 will be a constant difference between the preceding series of values 

 and those which follow, because the arbitrary zero from which I is 

 measured is now changed. For these reasons it is necessary to take 

 a new set of observations to serve as a standard of reference. 



Sir John Conroy has attempted to determine the effect of polishing 

 a crystal of Iceland spar by observing with a simple analyser the 



* < Phil. Trans.,' 1815. 



