and some suggested Uses of the Finest Threads. 495 



in a variety of threads, but an unforeseen difficulty arose which 

 I cannot yet explain. When the cell was moved round slightly 

 so as not to touch the mirror, the mirror moved at first in the 

 same direction as was to be expected, but it came to rest in a 

 new position, to reach which it had to move in the opposite 

 direction to the movement of the cell. Whichever way the 

 cell was shifted, the mirror always went the other to find its 

 position of rest. Thinking that it or the cell were electrified, 

 I damped both by breathing on them, but with no result, and 

 the next day the same effect was observable. So great was this 

 effect that I could set the cell with greater accuracy by 

 watching the spot of light than by the pointer carried by the 

 cell working over a 4-inch circle. 



Thinking that magnetism might have something to do with 

 this effect, I brought a horseshoe- magnet near the mirror, 

 when it was instantly deflected through a large angle. An 

 examination of the cement used (Loudon's bicycle cement) 

 showed that it was magnetic. Of many cements examined, 

 sealing-wax was more nearly neutral than any other. Bicycle 

 cement and electrical cement were strongly magnetic ; all 

 others except sealing-wax strongly diamagnetic. The appa- 

 ratus was therefore taken to pieces and carefully cleaned. It 

 was put together with as small a quantity of sealing-wax as 

 possible, and the mirror was attached to a fragment of thin 

 pure copper wire, which again was fastened by a speck of 

 sealing-wax to the thread. Even then the same kind of 

 effect as that already described occurred. Still a magnet 

 deflected the mirror, but not so much, and the cell was 

 practically neutral ; yet, when the cell was turned a little, 

 the mirror changed its position of rest. 



Without pursuing this question further, I put a window in 

 the protecting tube and turned the mirror by means of a 

 small instrument passed up from below. Thus neither window 

 nor support were moved. A piece of spun glass nearly 9 

 inches long gave a period of oscillation to the mirror of 2*3 

 seconds about. A lamp and millimetre-scale were placed 50 

 inches from the mirror. As all the observations were 

 expressed in tenths of a millim., to about which extent they 

 can be trusted, it is convenient to employ one scale of numbers 

 of which one tenth millim. is the unit. One complete turn of 

 the mirror is very nearly 160,000 on this scale. If the mirror 

 is moved through 160,000 in either direction and held for one 

 minute, and then allowed to take its new position, the change 

 in the position of rest is as soon as it can be read about 370. 

 This is reduced in about three minutes to 110. If the mirror is 

 moved through three turns, 480,000 of the scale, and held one 



