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



diameter and 70 cm. long, provided with two lateral tubes of 

 brass furnished with glass windows for the entrance and exit 

 of the concentrated solar beam. The further end of the tube 

 was lined with black velvet, which is far superior to a smoked 

 surface. The observation window was carried on a brass 

 tube 2 cm. in diameter and 15 cm. long, which was soldered 

 into a hole which perforated the end plate of the tube. This 

 was to shield the observation window as completely as possible 

 from light reflected from the edges of the lateral tubes. The 

 tube was filled with air filtered through cotton, and a beam 

 of sunlight reflected from a silvered glass mirror focussed at 

 the centre of the tube by means of a double convex reading- 

 glass 15 cm. in diameter. The scattered beam was easily 

 visible, even in a well-lighted room; it was bluish in colour, 

 and was practically extinguished by a nicol properly oriented. 

 No motes were visible. The diameter of the solar image 

 formed by the lens was 4 mm., which gives us an area ratio 

 of image to lens of 1/1400, i. e. we have a layer of air 4 mm. 

 in thickness, illuminated by a beam of sunlight 1400 times as 

 intense as normal sunlight at the earth's surface. This is 

 to be compared with the intensity of the blue sky near the 

 zenith, considered as due to the illumination of five miles of 

 homogeneous atmosphere by normal sunlight. It is to be 

 remembered that the entire atmosphere if brought to normal 

 pressure would form a layer five miles in depth. 



As the sun was at 60° from the zenith at the time of the 

 experiment, I chose a point 30° .beyond the zenith for obser- 

 vation, in order to work with the rays scattered in a direction 

 perpendicular to the sunlight. 



A small flake of silvered plate glass with a razor edge was 

 mounted in front of, and close to, the observation window. 

 This reflected to the eye the -light from the selected part of 

 the sky, which was reflected from a large silvered mirror 

 placed in the shade just below the mirror which reflected the 

 sunlight to the condensing lens which illuminated the tube. 



A rotating disk of black cardboard 35 cm. in diameter, 

 mounted on the shaft of: a motor, and furnished with a very 

 narrow radial slit near its rim, was mounted close to the glass 

 sliver. By means of this the intensity of the sky light could 

 be reduced by narrowing the slit until it matched the intensity 

 of the scattered light in the tube. If the glass sliver is 

 yiewed from a suitable distance its razor edge is seen in focus 

 projected against the cone of scattered light in the tube, 

 the edge disappearing when the match is secured. This 

 occurred with a slit 0*2 mm. in width. The reduction in the 

 intensity of the sky light is given by the ratio of the slit 



