NICHOLS AND HULL. — PRESSURE DUE TO RADIATION. 595 



an angle 6 with the incident radiation, then Iq 9 A =: I^ cos 6 9 A. The 

 total amount of diffuse radiation =: / \ Iq cos 6 9 A, over the surface of 



the hemisphere = I ^ 2 tt r^ J^ cos ^ sin ^ 5^ = tt I^ r^. This integral 



is the amount of the diffuse radiation in Table X. The force, due to 

 radiation of intensity Ig 9 A, normal to the vane is Iq cos 6 9 A, and the 



total is equal to I ^ 2 tt r^ ig cos^ 6 s.in 6 96 ^= ^ ir Iq r"^. It is thus seen 



that of the diffuse reflection, two-thirds is effective as light pressure. 

 This increases the air-silver reflection coefficients by 0.9 per cent and 

 the glass-silver values by 1.3 per cent. The small glass rod d (Fig. 2), 

 not present in the reflection measurements, decreased the reflecting area 

 of the silvered surfaces in the pressure measurements by 1.54 per cent. 

 The air-silver values are thus decreased by 0.92 X 1.54 ::= 1.4 per cent, 

 and the glass-silver values by 0.78 X 1.54 = 1.2 per cent. The applica- 

 tion of these two corrections gives the final corrected coefficients in Table 

 X. The diffuse reflection of black coatings deposited by the method 

 used in blackening the silver disc was measured and computed in the 

 same manner as the diffused reflection from the vanes C and D. The 



o 



agreement found by Angstrom* between the diffuse reflection of matte sur- 

 faces for normal incidence and the cosine law was abundantly close for 

 the present purpose. Five determinations of this reflection were made 

 under different conditions and with different coatings. The values in 

 percentages of the incident beam were 4.4 per cent, 4.5 per cent, 4.2 per 

 cent, 4.6 per cent, and 5.2 per cent ; average, 4.6 per cent. Thus only 

 95.4 per cent of the incident beam was absorbed by the black coating on 

 the silver disc in producing the temperature increase observed. Hence 

 the true energy of the beam is equal to the observed energy divided by 

 0.954. 



The silver disc, diameter 13.3 mms., used in the energy measurements, 

 received long waves and scattered radiation which passed round and 

 through the light pressure vanes of diameter 12.8 rams. This amount 

 was experimentally determined for both thin and thick silver coatings in 

 order to approximate to the average condition of the coatings in the liglit 

 pressure measurements and it was found to average (a) through air, 

 1.40 per cent; (b) through red glass, 1.44 per cent; (c) through water, 



* K Angstrom, Wied. Ann., XXVI. 271 (1886). 



