^0 Messrs. UoscoG Q.nd BaxeiideW on the relative [Feb. 23, 



The values found for the fihit-glass rod experimented on were, m 

 grammes' weight per square centimetre, 



M = 614,330,000, 

 n =244,170,000, 

 k =423,010,000, 

 ff = -258. 



The mode of experimenting is somewhat similar to that by which 

 Kirchhoff investigated the value of a for steel and brass ; but there are 

 several points of difference, especially this — that the portion of the glass 

 rod, whose flexure and torsion are measured, is sufficiently distant from 

 the places where external forces are applied, to eliminate the local irregu- 

 larities produced by their application. 



II. " Note on the relative Chemical Intensities of direct Sunlight 

 and diffuse Dajdight at different altitudes of the Sun." .-By 

 Henry E. Roscoe^ F.H.S., and Joseph Baxi^ndelLj F.H.A.S. 

 lleceived February 8, 1866. 



The method of determining the chemical intensity of daylight described 

 by one of us* presents a convenient means of experimentally comparing 

 the intensity of the chemically active rays which reach the earth's hori- 

 zontal surface directly from the sun with that of the same rays reflected 

 from the atmosphere and constituting diffuse daylight. For this purpose 

 it is only necessary alternately to expose pieces of the standard sensitive 

 paper, according to the method described in the memoir above mentioned, 

 to the action of the total light of day, and to the diffuse daylight alone, 

 which is easily done by cutting off the sun's direct rays from the sensitive 

 paper, by throwing upon the paper a shadow cast by a small screen, hav- 

 ing an apparent diameter slightly greater than that of the solar disk. In 

 the first case the chemical intensity of the total daylight, in the second 

 that of the diffuse light is determined ; the difference between these two 

 observations giving the chemical intensity of the direct sunlight. As the 

 experiments which we have already made in this direction have led us to 

 conclusions differing altogether from those derived from theoretical con- 

 siderations concerning the relative chemical intensities of direct and diffuse 

 sunlight, we think that, although this investigation is incomplete, the results 

 are worthy of the attention of the Society. No direct photometrical de- 

 terminations of the relative intensity of sun and diffuse light have up to 

 this time been made ; but Clausius f has calculated this relation for vary- 

 ing altitudes of the sun, founding his calculations upon the hypothesis 

 (generally adopted hy meteorologists to explain the red tints of the morn- 

 ing and evening sky) that the diffused light is reflected, not from the par- 



* Bakenau Lecture, 18G5. Phil. Trans. 1865, p. C05. 

 t Poggendorff'ci 'Aunalcu,' 13d. Ixxii. p. 291. 



