202 Profs. A. Gray and J. .1. D<.bbie. 



After fixing the fibre and allowing the arrangement to stand undis- 

 turbed for a day or two to allow the cement to harden, the scale wjis 

 placed in position with its ends at equal distances from the mirror. 

 The zero position of the spot of light on the scale was observed, and 

 the sole-plate turned round to give a total turning of the lower end of 

 the fibre relatively to the upper of 360. The couple was kept applied 

 for thirty seconds, and then taken off gradually without any jerking 

 of the fibre. This prevented torsional oscillation of the fibre after the 

 removal of the couple. In fifteen seconds after the removal of the 

 couple a reading of the position of the spot of light was taken. From 

 this the angular deflection of the lower end of the fibre was obtained, 

 and the angle so measured, divided by the length of the fibre, gave the 

 residual twist. 



The following table gives the results in radians per centimetre of 

 the length of the fibre x 10 4 . The diameter of the fibre was found 

 by weighing a known length of it, and calculating from the known 

 density of the glass. 



No. of specimen. Diameter in centimetres. Residual twist. 

 XXI 0-0109 2-09 



XXII 0-016 1-84 



XXIII 0-010 0-93 



XXIV 0-0105 3-83 

 XXV 0-0115 7-19 



XXVI 0-015 5-41 



XXVII 0-0187 2-44 



XXVIII 0-0105 5-52 



XXIX 0-0142 0-83 



XXX 0-0183 0-8 



XXXI 0-0148 1-9 



XXXII 0-0191 2-8 



The experiments on Specimens XXI XXVII inclusive were made 

 in June and July, 1898, and it was then thought that there ap- 

 peared to be some connection between the resistances and the 

 residual twist of the fibres. But since XXI to XXIII were glasses 

 whose resistances were too high for measurement, it was of course 

 impossible to draw any numerical conclusion on the point without 

 examining more specimens. A very little reflection, however, showed 

 that no exact comparison was possible from this point of view, as the 

 residual torsion was no doubt influenced in a very marked degree by 

 the immediate previous history of the fibre. But it is noteworthy 

 that the residual twist is very low in the case of the Jena barium 

 glass and in Messrs. Powell's lead-potash glasses, viz., in XXIII, 

 XXIX, XXX, and is abnormally high for the two ordinary glasses, 

 XXV, XXVI. 



