40 



KANSAS ACADEMY OF SCIENCE. 



ol's prism necessary to bring about the equality of the two spectra was out of the 

 direct control of the observer, who thus relied upon his visual perception, entirely 

 unbiased by the knowledge that the setting which he was directing his assistant to 

 make differed in this respect or that from a looked-for result. The advantage of 

 such an arrangement in work of this kind is very great. The personal equation 

 in photometry of all kinds is very large, and in the photometry of the spectrum 

 it is especially troublesome. 



To further eliminate the personal element, my assistant, Mr. W. S. Franklin, du- 

 plicated nearly every series of observations. The result of our measurements upon 

 the reflexion spectrum of this block of magnesium carbonate is shown in Curve I, 

 Figure I. Abcisste are wave-lengths, the spectral regions being denoted by the po- 

 sitions of the chief Fraunhofer lines. Ordinates are intensities, that of the region 

 of the D line being taken as unity. A true white possessed of the same reflecting 

 power as this block of magnesium carbonate would have as its curve of intensities 

 the horizontal dotted line shown in the figure. Deviations from this line indicate, 

 for each portion of the spectrum, the variation of the pigment from true whiteness. 

 It will be seen from the figure that this specimen reflected a greater proportion of 

 the red and less of the wave-length shorter than D than a true white would have 

 done. Another specimen of the same substance gave a curve of quite the same 

 character. No. 2, Fig. I. Its color, in a word, was a red of an exceedingly small de- 

 gree of saturation, and the inspection of other whites has led me to the conclusion 

 that the same peculiarity is common to most white pigments, excepting when 

 viewed in very thin films. The curve of this specimen of magnesium carbonate 

 once obtained, the measurement of others was a comparatively easy matter. This 

 specimen was used as a working standard, with which the pigment to be measured 

 could be compared. 



A comparison of several other white substances revealed the presence of blue 

 adulterants. The addition of a very small quantity of ultramarine or indigo com- 

 pensates for the preponderance of the longer wave-lengths in the spectrum of these 

 pigments, and adds greatly to their brilliancy. 



The intensity curves of specimens of white paper, of plaster of paris prepared 

 for dentists' use, and of carbonate of magnesium, show, unmistakably, the presence 



of this blue coloring 

 matter. Although 

 to the naked eye the 

 only efifect was in- 

 creased brilliancy 

 of tone, the adulter- 

 ation unfits the sub- 

 stance altogether 

 for use as a standard 

 white. These curves 

 are shown in Figure 

 II. The reflexion 

 spectrum of the pa- 

 per possesses a well- 

 marked maximum 

 between the Fraun- 

 hofer lines F and 

 G, while the intens- 

 ity of the rays be- 

 yond the G falls off to an amount less than that belonging to a true white. The 



BCD Kb F G 



Figure II. — Intensity curves of adulterated whites. 



(1) Plaster of Paris. (2) Magnesium Carbonate. (3) Paper. 



