THE MECHANICAL ACTION OF LIGHT. 



689 



ferently. Fig. 4 represents the best form : a 5 is a glass tube, to 

 which is fused at right angles another narrower tube, c d ; the ver- 

 tical tube is slightly contracted at e, so as to prevent the solid stopper 

 d which just fits the bore of the tube from falling down. The lower 

 end of the stopper, d e, is drawn out to a point ; and to this is cemented 



Fia. 4, 



a fine glass thread, about 0.001 inch diameter, or less, according to the 

 torsion required.' 



At the lower end of the glass thread an aluminium stirrup and a 

 concave glass mirror are cemented, the stirrup being so arranged that 

 it will hold a beam,/^, having masses of any desired material at the 

 extremities. At c in the horizontal tube is a plate-glass window 

 cemented on to the tube. At h is also a piece of plate-glass cemented 

 on. Exhaustion is effected through a branch-tube, /?, projecting from 

 the side of the upright tube. This is sealed by fusion to the spiral 

 tube of the pump. The stopper d e and the glass plates c and b are 

 well fastened with a cement of resin and bees'- wax. 



The advantage of a glass-thread suspension is that the beam 

 always comes back to its original position. 



An instrument of this sort, perfectly exhausted and then sealed 

 off, is shown at work in Fig. 5. It has pith-plates at the extremities 

 of the torsion-beam. A ray of light from the lamp is thrown on to 

 the central mirror, and thence reflected on to the graduated scale. 

 The approach of a finger to either extremity of the beam causes the 



* Some of the glass fibres used in these torsion-balances are so fine that when one end 

 is held between the fingers the other portion floats about like a spider's thread, and fre- 

 quently rises until it takes a vertical position. 

 TOL. TII. 44 



