and the Blue Colour of the Sky. 427 



In this case his value found at 57° was at a point of the sky 

 only 10° away from the sun, and is undoubtedly somewhat 

 too large, as the sky in the vicinity of the sun shows an excess 

 brightness due, as we shall see presently, to diffraction by 

 foreign matter. I have a record of one measurement made 

 by Mr. Nietz with an illuminometer from an air-plane at 

 3000 feet, which gave the zenith sky an intensity of half 

 that of the horizon sky. 



Molecular Scattering in Directions nearly parallel to that 

 of the Exciting Beam. 



The most elementary theory shows that the intensity of the 

 scattered light in directions nearly parallel to that of the 

 exciting beam should be only double the value observed in 

 perpendicular directions. This follows from polarization 

 considerations, for the light scattered in the parallel direction 

 will be unpolarized, all of the components in the incident 

 beam contributing to the illumination. 



It is well known, however, that the light of the sky, even 

 on a clear day and on the top of a mountain, is enormously 

 brighter close to the san than in distant regions of the sky. 



At sea-level on a very clear da}' the light of the sky at 45° 

 from the sun has only about 5 per cent, of the intensity shown 

 close to the solar disk. This estimate was made by holding 

 a mirror which reflected 5 per cent, of the incident light 

 against the blue sky, and then observing the reflected image 

 of the sky close to the sun, the solar disk being just hidden 

 by a chimney. A very good intensity match Avas secured at 

 about half a solar diameter from the sun, though the colour 

 match was imperfect, the light from the region close to the 

 sun appearing yellowish white in contrast to the blue sky. 

 The mirror was an acute prism of glass with a knife edge, 

 one surface being painted with black paint. The reflexion 

 was observed in the glass surface, and measurements with a 

 photometer showed that, at the angles commonly employed, 

 the intensity of the reflected light was roughly 5 per cent, of 

 the incident intensity. This device was employed subse- 

 quently in other experiments, and will be referred to in 

 future as the 5 per cent, prism (4 per cent, reflexion from 

 the front surface and 1 per cent, from the back). 



While there appears to be no doubt but that the great 

 intensity of the light in the vicinity of the sun is due entirely 

 to diffraction by small particles, it appeared to be worth while 

 to examine the scattering by dust-free air in a direction 

 nearly coincident with that of the exciting rays. The expe- 

 riment was made with a tube of galvanized iron 3 metres 



