494 Mr. C. V. Boys on the Production, Properties, 



It might be possible to calculate what would be the appear- 

 ance presented by a cylinder of given refractive power, and 

 1, 2, 3, &c. tenths of a wave-length of any kind of light 

 in diameter, when seen with a particular microscope. By no 

 other means does it seem possible to find out what the true 

 size of the ends of these threads really is. 



2. Properties. 



I can at present say very little of the properties of these 

 very fine fibres ; I am now engaged with Mr. Gregory and 

 Mr. Gilbert in investigating their elasticity. The strength 

 goes on increasing as they become finer, that is, when due 

 allowance is made for their reduced sectional area, and it 

 seems to reach that of steel, about 50 tons to the inch in 

 ordinary language ; but on this point I have not yet made 

 any careful experiments. 



The most obvious property of these fibres is the production 

 of all the colours of the spider-line when seen in a brilliant 

 light. The most magnificent effect of this sort I have seen, 

 was produced by a thread of almandine. One of these the 

 length of the room, even though illuminated with gas-light 

 only, was glistening with every colour of the rainbow. In 

 attempting, however, to wind it up, it vanished before me. 

 It is of course only visible in certain directions. 



The chief value of threads to the physicist lies in their 

 torsion. Spun glass, as is now well known, cannot be used 

 for instruments of precision, because its elastic fatigue is so 

 great that, after deflection, it does not come back to the original 

 position of rest, but acquires a new position which perpetually 

 changes with every deflection. If left alone, this position 

 slowly works back towards a definite place more rapidly as it 

 is further from it. 



To compare threads made of different materials, I made a 

 flat cell in which a galvanometer-mirror, made by Elliot Bros., 

 might hang, being attached to the lower end of the thread. 

 The upper end was secured to a fixed support, and a fixed tube 

 protected the length of the fibre from draught. The cell, 

 which could be moved independently of the rest, was protected 

 by a cover. By means of a lamp and scale, the exact position 

 of rest of the mirror could be determined with great accuracy. 

 On turning the cell round as many times as might be desired, 

 the mirror was turned with it, and could be left any time in any 

 position. On turning the cell back again, the mirror was 

 allowed to come to its new position of rest, air-resistance of 

 the cell bringing about this result in a few swings. By this 

 means I hoped quickly and accurately to determine the fatigue 



