1887/ 



on Sunlight Colours. 



67 



presence of mist on these occasions, and this mist must be merely a 

 collection of intangible and very minute particles of suspended water. 

 In a distant landscape we have simply the same or a smaller quantity 

 of street-mists occupying, instead of perhaps 1000 yards, ten times 

 that distance. Now I would ask, What effect would such a mist have 

 upon the light of the sun which shone through it 9 



It is not in the bounds of present possibility to get outside our 

 atmosphere and measure by the plan I have described to you the 

 different illuminating values of the different rays, but this we can 

 do : — First, we can measure these values at different altitudes of the 

 sun, and this means measuring the effect on each ray after passing 

 through different thicknesses of the atmosphere, either at different 

 times of day, or at different times of the year, about the same hour. 

 Second, by taking the instrument up to some such elevation as that 

 to which Langley took his bolometer at Mount Whitney, and so to 

 leave the densest part of the atmosphere below us. Now, I have 

 adopted both these plans. For more than a year I have taken measure- 

 ments of sunlight in my laboratory at South Kensington, and I have 

 also taken the instrument up to 8000 feet high in the Alps, and made 

 observations there, and with a result which is satisfactory in that 

 both sets of observations show that the law which holds with artifi- 

 cially turbid media is under ordinary circumstances obeyed by sunlight 

 in passing through our air : which is, you will remember, that more 

 of the red is transmitted than of the violet, the amount of each de- 

 pending on the wave-length. The luminosity of the spectrum observed 



Fig. 2. 



Relative Luminosities. 



at the Eiffel I have used as my standard luminosity, and compared 

 all others with it. The result for four days you see in the diagram. 



F 2 



