4o6 



NATURE 



[July 19, 1917 



measurements, based on the photographs taken 

 in 1913, only twenty-one g-ave an altitude under 

 90 km., and only sixteen an altitude above 

 220 km., the highest being- 323 km. Nearly 70 



the fact that in 1910 he observed heights under 

 50 km. on several occasions. Moreover, unless 

 this proves to be the case, or auroras attain 

 much lower levels in Greenland than in Norway, 

 we must suppose Prof, Paulsen's 

 estimates to have been seriously 

 at fault. The frequent asso- 

 ciation of aurora with magnetic 

 disturbance gives an additional 

 interest to Prof. Stormer's work. 

 It is of obvious importance to 

 have exact information as to 

 the changes in progress in 

 aurora during the large move- 

 ments frequently shown on 

 magnetograms during magnetic 

 storms. 



C. Chree. 



Fig. I. — Aurora borealis, photographed simultaneously from Bossekop (right) and Store Korsiies (left) 

 on March ii, 1913, iih. 36m. G.M.T. Altitude of the lowest parts 93-99 km. The star is Deneb. 



per cent, of the heights ranged between g6 and 

 120 km. Prof. Stormer claims that, as regards 

 frequency of occurrence, the results show two 

 distinct maxima, one between loi and 103 km., 

 the other between lOs and 108 km. 



THE DYE PROBLEM AMONG 



THE ENTENTE POWERS. 

 Tf HE synthetic dye problem 

 -*■ as it presents itself to 

 the French chemist was admir- 

 ably stated by Prof. Auger 

 in an address delivered on 

 February 11 to the Societe 

 des Amis de I'Universite de Paris. 

 For more than forty centuries the 

 art of the dyer was restricted by the narrow choice 

 of available colouring matters. The ancients were 

 acquainted with only ten dyeing principles, 

 namely, Tyrian purple, madder, archil, weld, 

 Persian berries, anatto, woad, indigo, catechu, and 



Fig. 2.— Aurora boreal'S, photographed sitnultanebuMy from Bossekop (right) and Store Korsnes Cleft) on March 30, 1913, gh. 20m. G.M.T. 

 Altitude of left border about 120 km. The stars of Lyra in the background. 



The year 1913 was one of sunspot minimum, 

 and Prof, Stormer seems disposed to associate 

 sunspot minimum with low p>enetrating power in 

 auroral rays, and so great height in the lower 

 edge of aurora. This seems necessary to explain 



NO. 2490, VOL. 99] 



the tannins, the last two applied either alone or witb" 

 iron mordants. This short list represents a very 

 restricted range of colours, dyeing in various 

 shades of red, yellow, orange, blue, brown, and" 

 black. Green dyes were unknown to the dyers. 



