44 



KANSAS ACADEMY OF SCIENCE. 



by the real contrast already mentioned, until we accept as blue what is actually only 

 a pure white of a very low degree of brightness. 



Scarcely less important in photometry than the choice of our standard white, is 

 the selection of a suitable black. 



The ideal black, a substance which absorbs all wave-lengths completely, does not 

 exist; and all blacks attainable in practice are simply pigments of very small reflect- 

 ing power. A ^^ure white of this kind would furnish what might appropriately be 

 termed a neutral black, one, namely, which although possessed of some power of re- 

 flexion, would reflect all wave-lengths equally well. 



The value of such a substance would be great, and would increase in proportion 

 as its reflecting power was insignificant. All other blacks may be regarded simply 

 as colored pigments of low brilliancy. 



The superiority of lamp-black is so generally recognized, and its use in photom- 

 etry so common, as to render it desirable to gain as accurate a knowledge as possible 



of the character of 

 its spectrum. Its 

 transparency to 

 rays of the infra- 

 red regions has 

 long since been es- 

 tablished, and its 

 average reflective 

 power for light has 

 also been determin- 

 ed. Its spectrum, 

 even when illumi- 

 nated by direct sun- 

 light, is so faint as 

 to make its photo- 

 metric analysis 

 quite unsatisfac- 

 tory, but the meth- 

 ods already applied 



to other pigments were found sufficient to give a tolerable indication of its character. 

 Fig.V shows the intensity curves of the spectrum of two specimens of lamp-black. 

 The deviations from the straight line, which would be characteristic of a neutral 

 but imperfect black, are such as enable us to define lamp-black, not as a white of di- 

 minished brightness, but as a purple in which red is the dominant hue, but in which 

 violet also greatly preponderates over the wave-lengths which form the central region 

 of the visible spectrum. Fortunately, none of the visible rays reflected by this pig- 

 ment are of sufficient intensity to prevent its adoption as the standard black in most 

 spectrophotometric work, but the character of its intensity curves is such as to war- 

 rant the suspicion that black pigments will be found, upon analysis, to differ as 

 widely from the ideal standard as do those substances which we are in the habit of 

 regarding as white. 



Univeubity of Kansas, November, 1886. 



5 



D 



H 



: F G 



Figure V. — Intensity curves of the reflexion spectrum of two speci- 

 mens of Lamp-black. 



