1920] on The Blue Sky and the Optical Properties of Air 157 



would have been observed with an equally good image of the 

 daylight sky. The part of the sky examined was near the pole, and 

 therefore nearly at right angles to the sun. If, as seemed possible, 

 the night sky derived its light from an attenuated atmosphere so 

 high as to be outside the earth's shadow, we should expect it to show 

 the same polarisation as the day sky. Since it does not do so, we 

 must attribute the light at night to some different origin. 



I was fortunate in being able to interest Prof. Hale in this matter 

 while he was on a visit to England, and as a result Mr. Babcock 

 repeated the observations in a modified form at the Mount Wilson 

 Observatory in California. The traces of polarisation which he 

 obtained in that clear atmosphere were even less than what I got in 

 England. 



Ozone, and the Limit of the Solar Spectrum. 



Although, as we have seen, the idea that the blue colour of the 

 sky is due to any action of ozone cannot be admitted, yet there are 

 points of great optical interest connected with the presence of this 

 gas in the atmosphere. We may now turn to the consideration of 

 some of these. 



It is of course well known that when the solar spectrum is 

 formed by a prism of quartz, or by a grating, the spectrum can be 

 observed to extend beyond its visible limit in the violet into the 

 region called ultra-violet. When, however, we examine the spectrum 

 of an electric arc (and for this purpose an iron arc is particularly 

 suitable), the extension is observed to be very much greater than in 

 the solar spectrum. This is not because the sun does not emit any 

 rays of the kind in question, but because the earth's atmosphere will 

 not allow them to pass through so as to reach us at the earth's 

 surface. There are many reasons for feeling sure that this is the 

 true explanation, but one of the simplest will here suffice. When 

 the sun is near the horizon, so that the rays pass obliquely through 

 the earth's atmosphere, and consequently have to traverse a thicker 

 absorbing layer, the extent of the ultra-violet spectrum is found to 

 be even less than when the sun is high and less air is traversed by 

 the rays. This sufficiently proves the point. 



It has long been suspected that ozone in the atmosphere is the 

 effective cause of this absorption of the ultra-violet rays. The most 

 important constituents of air, oxygen and nitrogen, do not appreciably 

 absorb at the point where the solar spectrum ends, nor do the con- 

 stituents of secondary importance, carbonic acid, water-vapour and 

 argon. We must therefore look to some rare constituent of air 

 which is very opaque to this region of the spectrum. Ozone possesses 

 this opacity, as I shall now show you. So far as I know it has not 

 been attempted to show this before to an audience, but I think you 

 will be able to see it without difficulty. As a source of light an 

 iron arc is used, and the lenses and prism used in forming the 



