PRESIDENTIAL ADDRESS, 885. 
particle in passing through a magnetic or electric field, and to determine 
the deviation of each particle from a rectilinear path due to encounters 
with molecules of matter. We can determine directly the mass of each 
a particle, its charge, and its velocity, and can deduce at once the number 
of atoms present in a given weight of any known kind of matter. In the 
light of these and similar direct deductions, based on a minimum amount 
of assumption, the physicists have, I think, some justification for their 
faith that they are building on the solid rock of fact, and not, as we are 
often so solemnly warned by some of our scientific brethren, on the shifting 
sands of imaginative hypothesis. 
The following Papers were read :— 
1. Preliminary Note on the Pressure of Radiation against the Source. 
The Recoil from Light. By Professor J. H. Poynrine, F.B.S., 
and Guy Bartow, D.Sc. 
All previous experiments on the pressure of radiation have been made 
on the force exerted by the radiation on a receiving surface. In the 
experiment described the pressure against the source from which the 
radiation starts is shown to exist. The effect may be termed the recoil 
from light. 
If an exceedingly thin disc, with both surfaces perfectly black, could 
be suspended in a perfect vacuum, a beam of energy P per c.c. directed 
normally on to it would exert pressure P when the temperature of the disc 
reached a steady state. For the energy absorbed at the front surface 
would, with a sufficiently thin disc, be emitted equally from front and 
back, and the resulting pressures of the issuing radiation would be equal 
and opposite, leaving only the pressure P of the incident beam. If the 
front surface were perfectly black and the back surface were perfectly 
reflecting and non-radiating, all the energy absorbed would be emitted from 
the front surface. As it would be emitted according to the cosine law the 
pressure due to it would be 3 P, and the total pressure would be § P. 
The experiment consisted in approximating to these conditions as nearly 
as possible. A globe was evacuated to as high a degree as could be attained, 
and in it was suspended by a quartz fibre a system of four discs, each 
12 cm. diameter, and at an arm 1cm. Each disc consisted of two cover- 
glasses, with a thin layer of asphaltum between. For a reflecting surface 
the outer surface of the glass was silvered by cathode deposition. The 
four discs were respectively black-black, black-silver, silver-silver, and 
silver-black on their front and back surfaces. A beam was directed on 
to the front surfaces of each of these in turn. The measurement of its 
energy per c.c. showed that on a perfectly black surface it should give 
142 divisions deflection of the scale used. On the actual discs it gave 
respectively 16°3, 225, 298, and 272 divisions—means of several deter- 
minations. The black surface was not a full absorber, partly through the 
reflection from the front glass. A small amount of radiometer action still 
remained, and both of these conditions tended to increase the pressure on 
it. The silver’ was not a perfect reflector, and so the second effect was 
slightly reduced. With perfectly black and perfectly reflecting surfaces 
the deflections should have been 14°2, 23°7, 28°4, and 284 divisions respec- 
tively. The observed results are sufficiently near to these to show that the 
increased effect on the second surface was chiefly due to the recoil from th 
radiation issuing from it as source. 
2. Some Properties of Light of very Short Wave-Lengths. 
By Professor THroporre Lyman.—See Reports, p. 132. 
1909. cc 
