8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 60 



(2) The effect increased during July, and had reached its maxi- 

 mum for Bassour by the middle of August, but seemed still increas- 

 ing at Mount Wilson to the end of August. 



(3) The maximum decrease of the total solar radiation attribut- 

 able to the haze seems to have reached nearly or quite 20 per cent at 

 each station. 



(4) It would appear from inspection of the transmission co- 

 efficients for different wave-lengths that the peculiarities of the sky 

 in the latter part of June produced no marked decrease of the 

 transparency of the air. 



(5) As regards the different wave-lengths of the spectrum, the 

 effect of the haziness is greater for visible rays than for infra-red 

 ones, but the difference of transmission does not increase so greatly 

 towards the violet as one would expect. Indeed for the Bassour 

 results there was nearly uniform effect throughout the whole visible 

 and ultra-violet spectrum. The Mount Wilson results, however, 

 show somewhat increasing effects towards the shortest wave-lengths. 

 This circurhstance is very noteworthy and indicates to us that the 

 size of the particles which produced the scattering of light was on 

 the whole much greater than the size of the particles which produced 

 the ordinary blue light of the sky. Lord Rayleigh has shown that 

 for particles small as compared with the wave-length of light the 

 scattering effect is inversely proportional to the fourth power of the 

 wave-length. It is from this extraordinarily rapid increase of the 

 scattering towards the shorter wave-lengths that we owe the very 

 blue character of the sky. The haze, on the other hand, produced 

 a whitish appearance on account of the larger size of the particles 

 concerned. 



As Bassour is at a lower altitude (1,160 meters) than Mount 

 Wilson (1,730 meters), it may be that the atmosphere above Bas- 

 sour included on the average grosser particles than the atmosphere 

 above Mount Wilson. This may account for the fact that the haze 

 effect for Bassour did not increase toward short wave-lengths as 

 rapidly as that for Mount Wilson. There is even a tendency toward 

 smaller effects at Bassour for the extreme ultra-violet than for the 

 visible rays. If this be real we think it may be due to a selective 

 action of the dust due to its composition of volcanic glass and 

 sulphur. 



Mr. Fowle collected on Mount Wilson some dust which fell upon 

 the mirror of the coelostat, and this dust has been very kindly 



