NINETEENTH ANNUAL MEETING. 



121 



(1) 



(2) 



.(3) 



B C 



E F 



Fig. 6. 



white paper, which had been treated in the process of manufacture with an almost 

 infinitesimal amount of some blue pigment for the purpose of counteracting the 

 slightly yellowish cast 

 common to these sub- 

 stances in their natu- 

 ral state, and to give 

 them brilliancy, were 

 subjected to analysis 

 with the spectropho- 

 tometer. It will be 

 seen from the curves 

 obtained from their 

 spectra (Fig. 6), that 

 they all vary quite as 

 much from true white- 

 ness as does the sky, 

 and yet they had been 

 selected as uncom- 

 monly pure specimens 

 of white pigments in 

 my search for a suit- 

 able standard of comparison; and the existence of the blue adulterant had been en- 

 tirely unsuspected until revealed by measurements with the spectrophotometer. 



The "artificial sky" of Lord Rayleigh and the beautiful blue blowpipe films of 

 antimony oxide, so familiar to the chemists when studied with the spectropho- 

 tometer, exhibited the same peculiarity as the azure of the true sky, excepting that 

 these opalescent blues possessed very faint spectra which differed less from that of 

 an absolutely white substance than any of the objects which I had analyzed. 



Was Leonardo da Vinci right, then, in deeming the azure an optical illusion, or 

 as we should now call it, a "subjective blue"? I know of no other interpretation of 

 the results of the spectrophotometric analyses to which I have just alluded. The 

 subject is fraught with great experimental difliculties, and we have still almost every- 

 thing to learn concerning the color of the sky. 



Should these measurements meet with final corroboration, and the subjective 

 character of the azure be thereby established, two important elements in the pro- 

 duction of the blueness of the sky will be found. In the first place, a true white is 

 much bluer than the standards of whiteness which we have been forced to adopt in 

 the absence of anything better; and in the second place, the well-known sensitive- 

 ness of the eye to the violet components of all colors of low intensity increases this 

 blue of contrast to an extent which we little realize.^ That these two factors are 

 always at work intensifying the azure, is already beyond question. To assert that 

 they are the only important factors, and that the excess of violet rays in the spec- 

 trum of light from the sky plays no essential part in the production of its color, 

 upon evidence derived from a limited set of experiments like my own, would be pre- 

 mature. We must await more exhaustive researches at the hands of those competent 

 to assail successfully this great problem, before we can hope for a final decision be- 

 tween the various theories which I have attempted to touch upon this evening. 



Univebsity of Kansas, November, 1886. 



'See Albert; Annalen der Physlk und Chemie, N. F. 16, p. 129. 



