COMPOSITE YELLOW* BY JACOB PAPISH AND F. E. AGEL
In 1871 while participating in a discussion on the existence of composite yellow, Lord Rayleigh’ described a method for the production of such a color. The method consisted in bringing together in a glass cell an alkaline solution of litmus and a solution of potassium chromate. “A mixture of these two liquids zn proper proportzons, easily found by trzalJ2isolates the green and extreme red rays with considerable perfection, and exhibits in a high degree the phenomenon of dichromatism.” Lord Rayleigh further stated that another liquid which answers the same purpose can be prepared by mixing solutions of chromium chloride and potassium dichromate. Experiments were carried out with the solutions suggested and the following difficulties were encountered : Mixtures, the concentration of which had been adjusted to absorb spectral yellow light, were distinctly red in color. If on the other hand the mixtures were diluted so that they looked yellow by transmitted light, they were found to be transparent to spectral yellow. Unsatisfactory results were similarly obtained when instead of mixing the two solutions they were placed each in a glass cell and the combination held in the front of a suitable light. I t is known that compounds of neodymium exercise strong absorption in the yellow spectral region. A solution of neodymium chloride containing approximately an equivalent of 4 gm. of Kd203 per I O O cc. was prepared and placed in a cell z cm. thick. This was joined with the aid of rubber bands to a 3 cm. cell filled with a saturated solution of potassium chromate. The combination when viewed by transmitted daylight appeared distinctly yellow. The same color was observed when diffused light from an incandescent bulb was employed instead of daylight. I t was not difficult at all to notice with the aid of a pocket spectroscope the absence of spectral yellow. This fact can be corroborated strikingly in the following manner: If on the grid of a Meker burner a piece of copper wire and a small lump each of a salt of sodium and lithium be placed in a row and the burner lighted, the flame when viewed with the naked eye will be seen to consist of three streaks colored green, orange-yellow and red, respectively. Now if the cells with the solutions of neodymium chloride and potassium chromate be placed between the flame and the eye of the observer the yellow light will be absorbed and only the red and the green streaks will be vi-ible. I n repeating this experiment it should be borne in mind that if too much of the sodium salt is ‘This paper is a natural corollary of experiments supported by a grant to BIessrs. Bancroft. Charnot and Xchols from the Heckscher Foundation for the Advancement of Science, established by August Heckscher a t Cornel1 Univarsity. 1 Nature, 111, 234 ( 1 8 7 1 ) . * The italics are ours.
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volatilized the orange-yellow light will be correspondingly intense and it will not be totally absorbed by the combination of liquids but merely reduced in intensity. While the work described in this article was in progress, there was in our laboratory a student who suffered from a peculiar kind of color blindness: She was insensitive to green, but saw all the rest of the spectrum. She was able to distinguish between the composite yellow and the yellow of a potassium chromate solution, though no one else in the laboratory could. This seems to indicate that certain individuals do have a primary sensation of yellow. When the cell combination with the two liquids was held against white light and the student was asked about the color, she declared it was “brownish red.” Cornell I‘niuerszty.
