1883.] 



Atmospheric Absorption. 



before the slit of the spectroscope : another point is that the Fraun- 

 hofer lines in certain regions (say A, 9420 to X 9800) are so irregularly 

 distributed as to preclude the idea that they all belong to the absorp- 

 tion of aqueous vapour, yet all are equally darkened by the band, 

 aud they do not spread out as the darkness of the band increases. 

 This is against the view of the bands being formed by aqueous vapour, 

 as we know it. 



The question then arises as to what these " water bands " can be 

 due — if not due to vapour. This we consider an open question, and 

 one which should be discussed. All we can state is that the absorp- 

 tions shown are similar to those of water (liquid) and they do not 

 seem to point to the watery stuff existing as vapour,* if we take the 

 visible spectrum as a guide. An intense blue sky at sea-level is often in- 

 dicative of moisture in the atmosphere, and it also seems to be indicative 

 of finely suspended matter of some kind. If this be the case, can this 

 suspended matter be suspended water stuff ? for if it be not, there is 

 no reason why the sky should be bluer on a moist day than on a dry 

 day. We would remark that the deep blue sky at sea-level is of a 

 different colour to the black-blue of high altitudes where, if they exist, 

 the fine suspended particles would be largely diminished in number, 

 aud the coarser particles which cause white haze would also be fewer. 

 The great difference of the intensities of the light from the blue sky 

 in England and at 10,000 feet was determined by one of us and 

 communicated to the British Association at Southampton, and the 

 enormous disparity between the two has some bearing on the question 

 we have been discussing. 



Addendum. 



In the above paper we have described the absorption due to " water 

 stuff " in the atmosphere to A, 9800, as it is only to that wave-length 

 to which the normal spectrum has been as yet published. We wish, 

 however, to add that there are bands commencing at A, 9800, X 12200, 

 and X 15200, f giving step by step absorption from the one wave-length 

 to the next, as in the diagram, which also correspond with cold water 

 bands. The absorption in the locality from 12200 downwards is 

 usually total, and it is only on dry cold days or at high altitudes that 

 we have noticed that rays of sufficient amplitude can penetrate to 

 cause photographic impression to be made. — March 24, 1883. 



* Unless it be held that the water itself holds vapour in solution. — March 12. 

 f These wave-lengths have been taken from the map of the prismatic spectrum 

 illustrating the Bakerian Lecture, 1880, and are approximate numbers only. 



G 2 



