THE PRESSURE DUE TO RADIATION. 127 



THE BALLISTIC OBSERVATIONS. 



In passing' from the static to the ballistic observations it mitst 

 alwa^'s be possible to compute the static equivalent of the ballistic 

 swings. Furthermore, the exposures should be made as short as pos- 

 sible without reducing the size of the swing below a value which can 

 ))e accurateh" measured. 



If the exposure lasts for one-half the period of the balance, the 

 deflection, if the gas action he small, and the damping' zero, is equal 

 to 26* where B is the angle at which the torsion of the fiber will 

 balance the moment produced by the radiation pressure. If the dura- 

 tion of the exposure be one-quarter of the period of the balance, the 

 angle of deflection is & s/~- The deflection is thus reduced b}' 30 per 

 cent, but the effect of the gas action is reduced in greater porportion. 

 It was decided, therefore, to expose for six seconds, one-fourth of 

 the balance period. Neglecting the gas action, but taking account of 

 the damping of the sj^stem, it may be shown that the total angle of 

 deflection of the torsion balance in the ballistic measurements is equal 

 to 1.357 times the angle at which the torsion moment balances the 

 moment of the radiation pressure. 



To make sure that the observed radiation pressures depended onl}^ 

 on the intensity of the beam, and were iminfluenced l\y the wave length 

 of the incident energ}', the l)allistic observations of pressure, the ther- 

 mal measurements of intensity, and the determination of the reflection 

 coefHcients were carried out for three eutirelv difl'erent wave g'roups 

 of the incident radiation. In the measurements designated ""through 

 air" uo absorbing medium was introduced in the path of the beam 

 between the lamp and the balance except the glass lenses and plates 

 alread}' mentioned. In the measurements ''through red glass" a plate 

 of ruby glass was put in the path of the beam between h., and d^ (tig. 2). 

 For the observations "through water cell" a 9-mm. Ia3'er of distilled 

 water in a glass cell was placed in the path of the l)eani at the same 

 point." 



Applying reduction factors to the averages in the separate series of 

 measurements of radiation pressure, we And the pressure of the stand- 

 ard light beam which has passed 



(a) through ah- to be (7.01 i 0.023) X lO-^^ dynes; 



(6) through red glass to be (6.94 ± 0.024) X 10"* .lynef^; 



(c) through water cell to be (6.52 ±0.028) X lO"'^ dynes. 



« Hero follow in the original paper detailed results of 14 separate series uf measure- 

 nients "through air," 8 "through water," and 9 "through red glass." 



