THE MECHANICAL ACTION OF LIGHT. 



269 



be again repeated, the differential action of tbe light on the black and 

 white surfaces keeps the bar moving, and the result will be rotation. 



Here is such a pith-bar, blacked on alternate sides, and sus- 

 pended in an exhausted glass bulb (Fig. 7). I project its image on 

 the screen, and the strong light which shines on it sets 

 it rotating with considerable velocity. Now it is slacken- 

 ing speed, and now it has stopped altogether. The bar 

 is supported on a fibre of silk, which has twisted round 

 till the rotation is stopped by the accumulated torsion. I 

 put a water-screen between the bar and the electric light 

 to cut off some of the active rays, and the silk untwists, 

 turning the bar in the opposite direction. I now remove 

 the water, and the bar revolves rapidly as at first. 



From suspending the pith 

 on a silk fibre to balancing it 

 on a point the transition is 

 slight ; the interfering action 

 of torsion is thereby removed, 

 and the instrument rotates 

 continuously under the influ- 

 ence of radiation. Many of 

 these little pieces of appara- 

 f \. ^s^ / \ ^^r tus, to which I have given 



jmr~ ~ ] ) tbe name of radiometers, are 



on the table, revolving with 

 more or less speed. The dia- 

 gram (Fig. 8) shows their 

 construction, which is very 

 simple. They aVe formed of 

 four arms of very fine glass, 

 \ / supported in the centre by a 



Y V needle-point, and having at 



FlG , 7 , Fig. 8. the extremities thin disks of 



pith lampblacked on one side, 

 the black surfaces all facing the same way. The needle stands in a 

 glass cup, and the arms and disks a.e delicately balanced so as to re- 

 volve with the slightest impetus. 



Here are some rotating by the light of a candle. This one is now 

 rather an historical instrument, being the first one in which I saw ro- 

 tation. It goes very slowly in comparison w r ith the others, but it is 

 not bad for the first instrument of the sort that was ever made. 



I will now, by means of a vertical lantern, throw on the screen the 

 projection of one of these instruments, so as to show the movement 

 rather better than you could see it on the table. The electric light 

 falling vertically downward on it, and much of the power being cut 

 off by water and alum screens, the rotation is slow. I bring a candle 



