Scientific American 12, 29.11.1856
Sir John Herschel first brought publicly into notice the fact that certain solutions appear of a different color according to the quantity seen through. The water of the ocean, for example, when lifted in a common tumbler, is clear and transparent - colorless; but looking down into a great body of it, as in the Gulf stream, it appears of a deep indigo color.
There are also certain varnishes, one coat of which is of a light brown color, but successive coats laid on the top of one another assume a black appearance. Dr. Gladstone read a paper on this subject before the late meeting of the British Scientific Association, of which the following is an abstract:
"A dichromatic solution was examined by placing it in a wedge-shaped glass trough, held in such a position that a slit in the window shutter was seen traversing the varying thickness of the liquid. The diversely colored line of light thus produced was analyzed by a prism; and the resulting spectrum was represented in a diagram by means of colored chalks on black paper, the true position of the apparent colors being determined by the fixed lines of the spectrum. In this way the citrate and cornenamate of iron, sulphate of indigo, litmus in various conditions, cochineal and chromium, and cobalt salts were examined and represented. Among the more notable results were the following: - A base, such as chromic oxyd, produces very nearly the same spectral image with whatever acid it may be combined, although the salts may appear very different in color to the unaided ete. Citrate of iron appear green, brown, or red, according to the quantity seen through. It transmits the red ray most easily, then the orange, then the green, which covers the space usually occupied by the yellow; it cut off entirely the more refrangible half of the spectrum. Neutral litmus appears blue or red, according to the strength or depth of the solution. Alkalies cause a great development of the blue ray; acids cause a like increase of the orange, while the minimum of luminosity is altered to a position much nearer the blue. Boracic acid causes a development of the violet. Alkaline litmus was exhibited so strong that it appeared red, and slightly acid litmus so dilute that it looked bluish purple; indeed, on account of the easy transmissibility of the orange ray through an acid solution, the apparent paradox was maintained that a large amount of alkaline litmus is of a purer red than acid litmus itself."
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