REPULSION RESULTING EROM RADIATION. 
519 
below it, was arranged the balance-tube c, having fixed above it a lens d, so that one 
of the pith balls was in the focus of the lens. A small right-angled prism ( e ) was then 
held in the different parts of the spectrum, so as to reflect any desired ray on to the lens 
and concentrate it on to the pith ball. By turning the right-angled prism sideways, 
the ray could be thrown off and on the ball, and the action readily noted. In the 
ultra-violet rays the repellent action was slight ; the effect increased as I made the 
reflecting prism traverse the spectrum from the violet to the red. The maximum 
action was in the extreme visible red, and it gradually faded away as the invisible rays 
beyond the red were reflected down. An appreciable action of repulsion was, however, 
observed a full half spectrum in length below the least refrangible visible rays. The 
plates of alum, interposed in the path of the rays, cut off a small portion only of their 
action. 
59. The following experiments were tried with a very sensitive pith-ball balance 
sealed up in a good Sprengel vacuum : — 
The beam was turned over, so that the centre of gravity was a very little above the 
centre of suspension ; the balance consequently set on either side. In this condition 
the ball touching the tube could be made to rise by placing the finger beneath it, the 
balance then oversetting on the other side. The higher ball could also be made to 
sink in the same manner by placing the finger on the top of the tube just over it. 
60. A piece of glass tube, bent in the form of a spiral (fig. 7), was supported on a 
stand, so that it could be slipped over one end of the balance-tube, which it fitted 
Fig. 7. 
loosely. Steam was then passed through the glass coil, and a woollen cover was put 
on it, so that the end of the balance-tube could be kept at 100° C. When the tempe- 
rature became uniform and steady the pith ball took up a position, as nearly as could 
be judged, in the axis of the spiral, and then remained stationary. On raising the hot 
coil, so as to bring the lower part close to the tube, the pith ball rose a little, and on 
lowering the spiral the pith ball sank. 
3 z 2 
