THE MECHANICAL ACTION OF LIGHT. 



261 



more rapid oscillation sideways. If I turn the leveling-screw so as 

 to raise the beam and weight, the nearer it approaches the horizontal 

 position the slower the oscillation becomes, and the more delicate is 

 the instrument. Here is the actual apparatus that I tried to work 

 with. The weight at the end is a piece of pith ; in the centre is a 

 glass mirror, on which to throw a ray of light, so as to enable me to 

 see the movements by a luminous index. The instrument, inclosed in 

 glass and exhausted of air, was mounted on a stand with leveling- 

 screws, and with it I tried the action of a ray of light falling on the 

 pith. I found that I could get any amount of sensitiveness that I 

 liked ; but it was not only sensitive to the impact of a ray of light, it 

 was immeasurably more so to a change of horizontality. It was, in 

 fact, too delicate for me to work with. The slightest elevation of one 

 end of the instrument altered the sensitiveness, or the position of the 



V 



Fig. 3. 



cs 



Fig. 4. 



zero-point, to such a degree that it was impossible to try any experi- 

 ments with it in such a place as London. A person stepping from 

 one room to another altered the position of the centre of gravity of 

 the house. If I walked from one side of my own laboratory to the 

 other, I tilted the house over sufficiently to upset the equilibrium of 

 the apparatus. Children playing in the street disturbed it. Prof. 

 Rood, who has worked with an apparatus of this kind in America, 

 finds that an elevation of its side equal to 360 ^ 0000 part of an inch is 

 sufficient to be shown on the instrument. It w T as therefore out of the 

 question to use an instrument of this construction, so I tried another 

 form (shown in Fig. 4), in which a fine glass beam, having disks of 

 pith at each end, is suspended horizontally by a line glass fibre, the 

 whole being sealed up in glass and perfectly exhausted. To the cen- 

 tre of oscillation a glass mirror is attached. 



Now, a glass fibre has the property of always coming back to zero 

 when it is twisted out of its position. It is almost, if not quite, a per- 

 fectly elastic body. I will show this by a simple experiment. This 

 is a long glass fibre hanging vertically, and having an horizontal bar 



