I 



OZONE IN THE UPPER ATMOSPHERE 449 



attain an important significance when it is considered that the 

 solar spectrum ceases suddenly at 293 fj,fi, indicating the 

 probability that light of shorter wave-length is absorbed in 

 the atmosphere. As the absorption of light by oxygen is not 

 appreciable for wave-lengths greater than 200 /jl/m, the above 

 phenomenon gives evidence of the presence of ozone in the 

 higher atmosphere. In addition to this behaviour of ozone 

 with respect to ultra-violet light, spectroscopic measurements 

 show that this gas gives two well-defined bands in the red 

 part of the spectrum. It is on account of this last absorption 

 that the gas possesses a marked blue colour by transmitted light. 

 Bearing of Ozone on the Colour of the Sky. — The view has 

 several times been put forward by chemists that ozone is 

 present in the upper atmosphere in sufficiently large amounts 

 to account for the normal blue colour of the sky. This idea 

 has not up to the present time been at all generally accepted, 

 but, on the contrary, in nearly all physical researches on the 

 optical properties of the atmosphere the presence of ozone 

 has been ignored. This omission has arisen on account of 

 the absence, until recently, of any definite quantitative measure- 

 ments of the amount of this gas in the air, and more especially 

 on account of the larger developments of the purely physical 

 theories which, on quite other lines, have established some 

 of the main factors which determine the nature of sky 

 light. 



On this physical basis it has been demonstrated by Tyndall, 

 and deduced from dynamical principles by Rayleigh, that one 

 factor which contributes to this colour is the presence of ultra- 

 miscroscopic particles of dust, which are present throughout 

 the atmosphere, and probably of meteoric and volcanic origin. 

 These particles, when of the same order of magnitude as the 

 wave-length of the light, exert a selective influence on the light, 

 causing the short waves which compose the blue light to be 

 reflected, while the longer waves, or red light, pass on. This 

 phenomenon has also been shown to operate in the production 

 of the greenish blue colour of glacier water and certain lakes. 

 The atmosphere is thus to be considered as a turbid medium ; 

 but this admission does not necessarily exclude other factors 

 which might contribute to the colour. 



After the development of the above theory to account for 

 the scattering of light, Lord Rayleigh drew attention to the 



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