770 The Bight Hon. Lord Rayleigh [Feb. 25, 



by Tyndall to become oblique, and the deviation is in the opposite 

 direction to that which would have been anticipated from the 

 Brewsterian law for the reflection of light from surfaces of finite 

 area. As I showed in 1881, the gradual precipitation of sulphur 

 from a very weak and acid solution of "hypo" exhibits the phenomena 

 remarkably well. At a certain stage, depending on the colour of the 

 light, the direction of maximum polarization becomes oblique. Even 

 when the obliquity is well established for blue light, red light still 

 continues to follow tlie simpler law, and the comparison gives cui'ious 

 information concerning the rate of growth of the particles. 



The preferential scattering of light of short wave-length in- 

 volves of course a gradual yellowing and ultimate reddening of the 

 light transmitted. The formation in this way of sunset colours is 

 well illustrated by the acid hypo. 



That Spring rejects this theory in favour of one which would attri- 

 bute sky-blue to absorption by oxygen or ozone, has been already 

 alluded to. Although one must not conclude too hastily from the 

 behaviour of these bodies when liquefied, it is, of course, possible that 

 their absorbing qualities may influence atmospheric phenomena in 

 some degree. But to attribute the blue of the sky to them seems out 

 of the question. It is sufficient to remark that the setting sun turns 

 7'ed and not blue. 



An interesting question remains behind. To what kind of small 

 particles — dispersing short waves in preference — -is the heavenly azure 

 due ? Tliat small particles of saline or other solid matter, including 

 organic germs, play a part, cannot be doubted, and to them may be 

 jittributed much of the bluish haze by which the moderately 

 distant landscape is often suffused. But it seems certain that the 

 very molecules of air themselves are competent to scatter a blue 

 light not very greatly inferior to that which we actually receive. 

 Theory allows a connection to be established between the transpar- 

 ency of air for light of various wave-lengths, and its known 

 refractivity in combination with Avagadro's constant, expressing the 

 number of molecules per cubic centimetre in gas under standard 

 atmospheric conditions. The first estimate of transparency was 

 founded upon Maxwell's value of this constant, viz. I'D x 10^^. 

 Recent researches have shown that this number must be raised to 

 2*76 X 10^^, and that the result is probably accurate to within 

 a few per cent.* It has been pointed out by Dr. Schuster that 

 the introduction of the raised number into the formula almost 

 exactly accounts for the degree of atmospheric transparency 

 observed at high elevations in the United States, apparently 

 justifying to the full the inference that the normal blue of 

 the sky is due to molecular scattering. But, altliough there is no 



* It is a curious instance of divergence in scientific opinion that while 

 some still deny the existence of molecules, others have successfully counted 

 them. 



