178 EXPERIMENTAL INVESTIGATION OF THE PRESSURE OB^ LIGHT. 



Without entering into the details of the experimental arrangements, 

 the principle of the method emplo3^ed ma}^ be briefly explained as 

 follows: A torsion thread hangs in a highl}^ exhausted l>ell jar and 

 carries a vertical glass rod. Thin disks of 5 mm. diameter, of the 

 metal to be investigated, are attached to this rod at a distance of 10 mm. 

 from its axis. If the radiation from an arc lamp is (concentrated on 

 one of the disks the incident radiation will exert a pressure upon it, 

 and it will retire until the pressure due to radiation is balanced b}' the 

 torsion of the glass thread; the angle of torsion is measured by a 

 mirror and scale as for a galvanometer. This observation permits the 

 determination of the absolute magnitude of the pressure (in d3nies) if 

 the directing force of the torsion thread is measured in absolute units 

 by one of the well-known methods. 



In order to compare the observed pressures with those computed, 

 according to Maxwell and Bartoli, from the amount of energy incident 

 and the reflecting power of the vane used, the same beam of light was 

 directed upon a circular aperture of exactly the same size as the 

 vane, and the rays passing through Avei-e caught by a calorimeter. If 

 we divide the quantity of energy incident per second, as measured by 

 the calorimeter, l)y the velocity of light, we obtain the amount of 

 pressure in dynes exerted by the light upon a perfectly absorbing 

 body, according to Maxwell and Bartoli. 



The measurements which were made repeatedly and with difl'erent 

 apparatus jdelded accordingly the following results: 



Vane used. 



Radiation 

 obsetved. 



Pressure 

 computed. 



Blacl? platinum plated 



Bright platinum 



Bright aluminum 



Bright nickel 



1.1 

 l.S 

 •2.0 

 ].l 



1.0 

 1.6 



l.H 

 1.6 



For such complicated and difficult measurements a better agree- 

 ment between observations and computation can not be (expected. A 

 discussion of the possil)le errors of o])servati(>n shows that they are 

 considerable, hut that within the limits of error the existence of the 

 light pressure of Maxwell and Bartoli are quantitatively confirmed. 



This result is of importance to astrophysics as furnishing a much 

 simpler explanation of the repulsive force of th(> sun than the hypo- 

 thetical ones of electrical charges. A tirm basis, amenable to compu- 

 tation and assured by experiment, is thus givcMi to the view expressed 

 by Kepler. 



Physical Institute, Mo.scoii^ Nominhor^ 1001. 



