38 KANSAS ACADEMY OF SCIENCE. 



be obtained commercially in cubes or rectangular blocks of convenient size and the 

 requisite degree of firmness, the surfaces of which are easily removed by scraping 

 as often as they become soiled. These advantages, coupled with the fact that it ap- 

 peared to the eye to be a very nearly perfect white, led to its adoption as a working 

 standard in my spectrophotometric study of pigments^ and of the color of the sky.^ 



An inspection of difiPerent commercial specimens of the carbonate, placed side 

 by side, discovered such differences of tint as to make it impossible to select the one 

 most suitable for a standard. The tendency is in such a case, to accept as white the 

 one presenting the most brilliant appearance, and to regard all others by compari- 

 son as yellowish or creamy, just as one is apt unconsciously to adopt the sharpest 

 of a series of musical tones of nearly the same pitch as the standard, and to regard 

 the rest as flat. 



In either case the result of the comparison is almost sure to be misleading. It 

 becomes necessary, therefore, to subject the specimens to a careful spectrophoto- 

 metric analysis, and to adopt that one as standard which shows the least deviation 

 from the true white. 



This determination is a matter of considerable difficulty. It involves the com- 

 parison, wave-length for wave-length, of the spectrum of the light reflected by the 

 pigment, with the direct spectrum of the light illuminating it. The intensities of 

 the direct and reflected spectra are so very different as to make this comparison 

 exceedingly unsatisfactory. Any reduction of the intensity of the direct spectrum, 

 by means of absorbing media or by reflection, is inadmissible, since we have no 

 substance at command which will absorb or reflect different wave-lengths in pre- 

 cisely the same proportion. The use of a revolving disk with slits of adjustable 

 width might be regarded by some as free from objection; but it is a matter of 

 serious question whether the comparison of an intermittent ray with a persistent 

 ray, even of the same wave-length, is in any degree trustworthy. 



A source of error, unavoidable in any form of spectrophotometer, viz., slight 

 modifications on the one hand by selective absorption and reflection in the Nicols 

 prisms and on the other hand in the prisms or mirrors by means of which the un- 

 polarized ray is brought to the slit of the collimator, cannot be eliminated. But 

 these effects are fortunately small, and may be allowed for when their amount has 

 been approximately determined. In the choice of the source of illumination one is 

 limited to diffuse daylight, which is unfortunately subject to most perplexing fluc- 

 tuations in intensity and quality alike, and to the use of the gas or petroleum flame 

 or of the incandescent electric lamp — sources which, at their best, have little to be 

 desired in the matter of constancy, but which are of such small intensity and so low 

 in temperature as to seriously affect the accuracy of the comparison, even in the 

 brightest parts of the spectrum, and to render it impossible to carry the measure- 

 ments much beyond the neighborhood of the G line. In spite of these disadvan- 

 tages, I found that the most trustworthy results were to be obtained by making 

 comparison in an otherwise dark room, using as the sole source of illumination the 

 flame of a "bat's-wing" gas burner. 



The spectrophotometer used in these measurements was of the form described 

 in the researches mentioned in a previous paragraph. In this instrument the spec- 

 tra to be compared are brought into vertical juxtaposition in the eye-piece of an 

 ordinary one-prism spectroscope, ecjual wave-lengths in the tube being everywhere 

 in the same vertical straight line. The brighter of the two spectra was polarized by 



» American .Journal of Science. Vol. 28, Nov., 1884. 



' Proceedings of tlie American Association for the Advancement of Science, Ann Arbor meeting, 

 1885. Vol. 34, p. 78. 



