150 Dr. Hopkinson on Torsional [Dec. 12, 



observed after-effect was not due to the fibre twisting within the 

 clamps and then sticking. The difficulty was easily avoided by em- 

 ploying two mirrors, each cemented at a single point to the glass fibre 

 itself, one just below the npper clamp, the other jnst above the lower 

 clamp. The npper mirror merely served by means of a subsidiary 

 lamp and scale to bring back the part of the fibre to which it was 

 attached to its initial position. The motion of the lower clamp was 

 damped by attaching to it a vane dipping into a vessel of oil. The 

 temperature of the room when the experiments were tried ranged from 

 13° C. to 13*8° C, and for the present purpose may be regarded as 

 constant. The lower or reading scale had forty divisions to the inch, 

 and was distant from the glass fibre and mirror 38J inches, excepting 

 in Experiment V, when it was at 37J inches. Sufficient time elapsed 

 between the experiments to allow all sign of change due to after-effect 

 of torsion to disappear. In all cases the first line of the table gives 

 the time in minutes from release from torsion, the second the deflection 

 of the image from its initial position in scale divisions. 



Experiment I. — The twisting lasted 1 minute. 



t 1 2 3 4 5 7 10 17 25 



Scale divisions . . 22 13 9 7 5| 4 3 2 1 



Experiment II. — The twisting lasted 2 minutes. 



t 1 2 3 4 5 7 10 20 40 



Scale divisions. . 38 25 18 15 13 10 8 4J 3J 



Experiment III. — Twisted for 5 minutes. 

 / 1 2 3 4 5 7 



Scale divisions 64 51 41 \ 35J 32 



10 15 22 58 15 



Scale divisions 21J 17 14 7 2 



Experiment IY. — Twisted for 10 minutes. 



t | 1 2 3 4 7 10 



Scale divisions .... 106 85 66 57 4<H 37J 31 



t 15 25 45 120 170 



Scale divisions .... 24J- 18 13 7 6 



Experiment Y. — Twisted for 20 minutes. 



t 1 2 3 4 5 7 10 



Scale divisions .... 110 89 75 68 61£ 52 44 



* 15 25 40 60 80 100 



Scale divisions 35| 26J 21 18 13J 12J 



