144 Intelligence and Miscellaneous Articles, 



curve to that which corresponds to an ordinate =100 for X=565. 

 All these curves are therefore comparable : 



X. 635. 600. 565. 530. 510. 



100(^!) 4 62-68 



78-63 100 128-1 150-6 



•o 



78-55 



?) 



130-3 



151-4 



76-00 



f> 



126-0 



146-0 



76-47 



5J 



141-1 



180-8 



^J 49-73 69-73 „ 146-7 180-7 



100 | (England) 58-59 



100 ? (Potsdam) 63-00 



^(SS): - 71.31 „ 130-0 „ 



We see, by a simple inspection of this table : — 



(1) That in England the blue of the sky observed by Lord 

 Eayleigh is more saturated than that which was observed at 



Potsdam by M. Vogel : it agrees sufficiently with the formula ~i 



while .giving a little deeper blue. 



(2) That the blue of the sky observed at Montpellier, during the 

 month of January 1890, is deeper than those observed in Euglaud 

 and in Germany, and that it approaches more nearly the formula 

 J_ 



I give below, as an example of the application of this method, 

 the formulae which give the value of 100 -^ for three observations 

 made by M. Houdaille and myself at the summit of Mt. Yentoux : 



h m "R /565\575 



Aug. 3, 1889, 10 40 . . | =100 (—) . Sky of a clear blue. 



Aug. 3, 1889, 10 20 . . £ = 100 (_J . Sky of a clear blue. 



Sept 3, 1889, 9 40 . . £ =100 (—J . Sky of a deep blue. 



The differences between the values found and calculated are of 

 the order of errors which might be made in photometric observa- 

 tions ; they are sometimes augmented in consequence of the 

 extreme variability of the blue tint of the sky under the influence 

 of causes most insignificant in appearance. — Comjptes Renclus, 

 May 25, 1891, p. 1176. 



