Nineteenth Annual Meeting. 



43 



(4) 



denser; and the solution, in which at first no selective absorption can be noticed, 

 transmits, as the experiment proceeds, light in which red and yellow become more 

 and more dominant. 



Fig. IV shows the intensity curves of the reflexion spectrum of the hyposulphite 

 solution in its blue stage and in the milky stage, also the absorption spectrum of 

 the latter. 



In the same figure is given, for purpose of contrast, an intensity curve obtained 

 from artificial ultramarine, the result of measurements which have been already 

 published.^ 



It will be seen upon inspection of these curves, that the sulphur precipitate, even 

 in its first stages, shows no measurable excess at the violet end of the spectrum; 

 that this curve differs from the denser precipitate only in intensity; and that both 

 curves represent within narrow limits the spectrum of the true white. 



The absorption spectrum of this solution (Curve 3) shows a decided excess in 

 the red and yellow, and an equally unmistakable deficiency toward the violet. 



The appearance of a pronounced and very beautiful blue in substances which re- 

 flect light of an almost pure white is, I believe, by no means confined to the cases 

 under considera- 

 tion here. Two im- 

 portant factors in 

 the production of 

 these subjective 

 blues may be briefly 

 referred to. In the 

 first place, we un- 

 consciously adopt 

 as our standard of 

 whiteness, light re- 

 flected by pigments 

 which are really of 

 a decidedly yellow- 

 reddish tone. The 

 excess of yellow and 

 red in the ordinary 

 so-called whites of 

 every-day life leads 

 us to the adoption 

 of this false stan- 

 dard, because, with 



the probable exception of snow, all white substances with which we have to do pos- 

 sess this peculiarity. 



Any substance reflecting all wave-lengths in the same proportion will be regarded 

 as blue by comparison, and whenever a white pigment appears in exceedingly thin 

 films against a black background, and its deviation from the true white by absorii- 

 tion disappears, another factor enters, the importance of which is, I believe, greatly 

 underrated. 



The increasing delicacy of the eye to the more refrangible rays of the spectrum 

 as the light reaching the retina diminishes 2, comes powerfully into play in the case 

 of these thin films, reinforcing an impression to which the judgment is already biased 



(3) 

 (2). 



B C D E F G H 



Figure IV. — Intensity curves of the spectrum of Sulphur precipitated 

 from a solution Hyposulphite of Sodium. 

 (1) Reflexion spectrum in the incipient (blue) stage. (2) Reflexion 

 spectrum in the "milky" stage. (3) Absorption spectrum of the 

 solution in the " milky " stage. (4) Reflexion spectrum of artificial 

 Ultramarine. 



* Spectrophotometric Study of Pigments, Am. Journal of Science. Vol. 28. 

 ''Albert; Wiedemann's Annalen. Vol. 16, p. 129. 



