48 Bemnrhs hi/ Mr. Bridges Lee on atmospJieric absorption. [Feb.^ 



all sliades of pale blue to white (as seen by reflected light) and the light 

 reflected was polarised as above stated. 



Naturally aqueous mist in the atmosphere is always accompanied by 

 aqueous vapour, but while the latter betrays its existence by selective 

 absorption, the former is detected by its diffusive and polarizing effects. 



Dust haze is more difficult to distinguish from aqueous mist, but it 

 may be noticed, as Mr. Blanford had remarked, that dust haze is most 

 plentiful in the atmosphere when aqueous vapour mist and cloud are least 

 plentiful. The effects of each may be capable of differentiation with tho 

 aid of a spectroscope and absorbing media. Dust haze is composed of 

 minute solid particles which reflect slightly and absorb strongly. The 

 absorption by dust haze is diffuse, being greatest at the most refrangible 

 end of the spectrum, but it extends with gradually diminishing intensity 

 through the green, yellow, orange and red. The absorption by clean aqueous 

 mist or cloud is quite different. 



A clean white cloud flits between the earth and the sun, and it is 

 found possible to turn one's eyes without pain toward the sun's rays. The 

 intensity of the rays which reach us is greatly diminished, but the sun still 

 looks white or nearly so through the cloud and the spectrum will differ in 

 little but intensity from the ordinary ' spectrum obtained on looking 

 directly at the sun before or after the cloud has passed. In look- 

 ing at the sun through a dust cloud (or a London or Calcutta fog, 

 which consists of watery globules with particles of solid dust or dirt 

 attached), the sun appears to pass from white through all the shades of 

 yellow, orange and red until its direct raj'-s are at last completely quenched^ 

 The effect of a clean cloud of water particles may be roughly compared to 

 the effect of a sheet of nearly colourless ground glass, the effect of dust 

 or dirty fog to a similar sheet of smoked glass. Further thought and a 

 close and careful attention to experimental detail will probably be necessary 

 before practical effect can be given to the suggestion here thrown out, but 

 Mr. Lee's object in making these remarks was to indicate the possibility 

 of increasing the value of certain classes of meteorological observations by 

 calling in the aid of the spectroscope and polariscope as meteorological 

 instruments. In all Mr. Lee's expeditions to the Alps and Himalayas, he 

 has invariably carried a small Nicol's prism, and has frequently found that 

 with its aid he has been able clearly to discern distant objects which were 

 otherwise rendered indistinct or invisible by the polarised light from inter- 

 vening mist. Professor Tyndal in one of his books or papers refers to 

 having employed a Nicol's prism for the same purpose. 



Mil. BLA]s:FOED,in replying to Mr. Bridges Lee's remarks, said he thought 

 that gentleman had misunderstood the nature of the difficulty he had 

 briefly alluded to in distinguishing between the effects of dust haze and 

 high diffused cloud. The remark had reference only to the actinonaetria 



