430 Prof. R. W. Wood on Light Scattering by Air 



shown by the dust-free air in the tube. This is about; what 

 we should expect, as the outside air was not free from 

 foreign matter, and as will appear from experiments to be 

 described presently, the scattering power of the air close to 

 the earth's surface is from two to three times the average 

 scattering power of the atmosphere taken as a whole. The 

 colour of the luminosity in front of the dark cave was 

 decidedly whiter than that seen in the tube, which also is to 

 be expected. This experiment makes it appear probable 

 that if the atmosphere were absolutely free from foreign 

 matter, the sky would be no brighter at the sun's edge than at 

 remote distances*. The performance of the long tube was 

 very satisfactory : it was made from oddments from the junk 

 heap, and the lens ground and polished from a piece of thin 

 plate glass. The silvered glass slivers referred to are made 

 by silvering a piece of plate glass, polishing the surface, and 

 then breaking off thin scales by striking the edge with a 

 hammer in a direction parallel to the silvered surface. The 

 razor edge oE one of these scales disappears when the photo- 

 metric balance is secured quite as completely as the dividing 

 line of a Lummer-Brodhun prism. The observation hole was 

 left clear, as it was found that a glass cover was apt to send 

 some light to the eye. 



Scattering of Air close to the Earth's Surface* 



This question was investigated by determining the depth 

 of the stratum of air in full sunlight (observed against a 

 black cave), necessary to give a luminosity equal to 5 per cent, 

 of the luminosity of the blue sky 60° from the sun on the 

 clearest days. 



A 4'5-inch astronomical telescope was directed towards 

 the black cave and the eyepiece removed. The real image 

 of the aperture of the cave was then examined with the 

 nigrometer, the 5-per cent, reflecting prism being held in 

 front of the small circular hole in such a position that its 

 thin edge bisected the aperture and reflected the light of the 

 blue sky to the eye. On slightly hazy days an intensity 

 match was secured with the cave at a distance of 400 feet, 

 while on the clearest day of all the distance increased to a 

 trifle over 1000 feet. Similar results were secured by observing 

 the open doors and windows of distant buildings with the 

 nigrometer without employing the telescope. 



In discussing these results the question again comes up as 



* Dr. Luckiesli and Mr. Nietz "both report having frequently observed 

 that at great altitudes (15,000 to 20,000 feet) the sky appears dark blue 

 right up to the edge of the solar disk. 



