1875.] The Mechanical Action of Light. 343 
I next tried experiments in which the rays of the sun, and 
then the different portions of the solar spectrum, were pro- 
jected on to the delicately suspended pith-ball balance. Jn 
vacuo the repulsion by a beam of sunlight is so strong as 
to cause danger to the apparatus, and resembles that which 
would be produced by the physical impact of a material body. 
A simpler form of the apparatus for exhibiting the phe- 
nomena of attraction in air and repulsion in a vacuum con- 
sists of a long glass tube, ab (Fig. 3), with a globe, c, at one 
end. A light index of pith, de, is suspended in this globe 
by means of a cocoon fibre. 
When the apparatus is full of air at ordinary pressure, 
Fic. 4. 
a ray of heat or light falling on one of the extremities 
of the bar of pith gives a movement indicating attraction. 
When the apparatus is exhausted until the barometric 
gauge shows a depression of 12 millims. below the barometer, 
neither attraction nor repulsion results when radiant light 
or heat falls on the pith, but when the vacuum is as good as 
the pump will produce strong repulsion is shown when radi- 
ation is allowed to fall on one end of the index. An appa- 
ratus of this kind constructed with the proper precautions, 
and sealed off when the vacuum is perfe¢t, is so sensitive to 
heat that a touch with the finger on a part of the globe near 
one extremity of the pith will drive the index round over go’, 
VOL..V. (N.S.) a 
