THE PRESSURE DUE TO RADIATION. 131 



and with different coatings. The values in percentages of the incident 

 beam averaged 4.6 per cent. Thus only 95 A per cent of the incident 

 beam was absorbed b}^ the black coating on the silver disk in produc- 

 ing the temperature increase observed. Hence the true energy of the 

 beam is equal to the observed energy divided by 0.954. 



The silver disk, diameter 13.3 mm., used in the energy measure- 

 ments, received long waves and scattered radiation which passed round 

 and through the light-pressure vanes of diameter 12.8 mm. This 

 amount was experimentally determined for both thin and thick silver 

 coatings in order to approximate to the average condition of the coat- 

 ings in the light-pressure measurements, and it was found to average 

 (a) through air, 1.40 per cent; {h) through red glass, 1.44 per cent; 

 {{') through water, 0.46 per cent. On this account the energ}^ E of 

 the standard radiation must be reduced by the above percentages." 



A comparison of observed and computed pressures follows: 



Observed values 

 in 10—5 dynes. 



Computed values 

 in 10— 5 dynes 



Differences. 



Through air 



Through red glass. 

 Through water 



"i>=7.01±0.02 

 p=6.94± .02 



7.05+0.03 

 6.86± .03 



p=6. 52± .03 ' ().-48± .04 



Per cent. 



-0.6 

 +1.1 

 - .6 



"The pressure and fucrgy measurements for the three different wave groups through air, red glass, 

 and water cell constitute three independent experiments. The values for yiressure. 7.01, (i. 91, and 

 ^52 in the three cases are only accidentally related. The difference arises from tlie different reflect- 

 ing power of the 4.5° glass plate (tig. 2) for the different beams and from the fact that the nnlieations 

 of the lamp galvanometer G.) connected with bolometer R were probably not strictly proportional to 

 energy for throws differing as widely as 33, (50, and 100, which, roughly, were the relative intensities 

 of the beams through water cell, red glass, and air. The function of the lamp bolometer and galva- 

 nometer was purely to keep a check on the small variations of the lamp, which rarely fluctuated more 

 than 10 per cent on either .side of the mean value. 



An estimate of the approximate magnitude of the gas action not 

 eliminated by the ballistic method of observation [of which details 

 appear in the original paper] shows that the uneliminated gas action, 

 b}^ the most liberal estimate, can not have exceeded 1 per cent of the 

 radiation pressure. Because of its smallhess and indeliniteness no 

 correction for gas action has been made to the final pressure values. 

 If corrections were applied, its effect would be to reduce slightly the 

 observed pressures. 



From the agreement within the probable errors of the air, red glass, 

 and w^ater values with the theory it appears that radiation pressure 

 depends onh^ upon the intensity of the radiation and is independent 

 of the wave length. 



Tlic Maxwell-Bartoli theory is thus (luantitatively confirmed within 

 the probable errors of observation. 



Wilder Laboratory, Dartmouth College, 



, Hanover, W. H., Febi-uary, 1903. 



(' As the average pitch of the cone of the incident beam was about 1 part in 40, no 

 correction need be appUed for inclination. Furthermore, the inside of the bell jar 

 was blackened and the zero of the balance was so chosen that energy reflected from 

 the window admitting the beam conld produce no pressure effects. 



