426 Journal of the Asiatic Society of Bengal. [NS., XIV, 
pressure depending on the small amount of reflection from the 
glass surface. 
The occurrence of the term c in the denominator makes 
the pressure extremely small. Let us take for example the 
rays of the earth is equivalent to 2°4 calories per minute. The 
pressure therefore 
24x42 10" 
3x 10'° x 60 x 981 
By using the are, or a very high candle power filament lamp 
(1500 wt/} wt. for example), and by concentrating the light 
by means of a lens of large aperture, the pressure can be it- 
creased to about 100 times But still it is extremely small. , 
It was for demonstrating the pressure of light that Crookes 
was led to invent his famous ‘‘radiometer.”’ As is well known, 
this consists of a delicate cross of glass or mica vanes SUS: 
pended on a pivot and enclosed within a glass cylinder from 
which air can be pumped off at will The alternate faces of 
the vanes.are covered with lamp-black. When light falls on 
the vanes it begins to rotate rapidly about the axis. __ 
Crookes was inclined to explain this motion as being due 
to the pressure of radiant energy, but Zéllner? showed that 
the effect observed was rather spurious, and exceeded theo- 
retical pressure by at least 108 times. He showed that the ee 
was really due to the unequal heating of the two sides of the 
vanes, 
gms. weight = "56 x 107" gms. weight. 
ment. Two thin discs of silvered or blackened glass, mr 
pumped out at will. A galvanometer mirror is att 
vertical part, with its plane at right angles to the plane | tion 
vanes. But with light incident on the vanes, the deer y 
observed was very irregular, and sometimes was completel 
in the wrong direction. of 
_ But in spite of repeated failures to detect the penne 
radiation, theoretical investigation had, in the meantime, nn 
advanced so far that it was not possible to deny its @ ced by 
e have seen that the pressure of light was eer an 
Maxwell from the electromagnetic theory of light. by ra 
argument involving the assumption of pressures a 
: Phil. Trans. 1874, Vols. 164, p. 501. 11} 
2 Pogg. Ann Bd. 160, p. 154, 1877 (suggested by Maxwell). 
8 Bartoli, Nuovo Cimento, 15 883. 
* Hull, Phys. Rev. May 1905 
»?p- 
