a B Red. the spectrum. The figure is based on measurements made by the author on a flint-glass prism, with aid of a large spectroscope, or rather spectrometer, admirably constructed by Wm. Grunow, of New York. At the same time a C series of observations was made on the extent of the coloured spaces in the spectrum; these are D indicated in the figure, and accurately given in one of the tables that follow.* Let us suppose that the spectrum from A to H includes 1,000 parts; then the E following table indicates the positions of the fixed lines: Red-orange. Green-yellow and Yellow-green. Green and F * It will be noticed that the term indigo, originally introduced by Newton, has been entirely rejected in this work, and ultramarine substituted for it. Bezold suggested this change some time ago, basing his objection to indigo on its dinginess; the author, however, finds a much more fatal objection in the fact that indigo in solution, and as a pigment, is a somewhat greenishblue, being really identical with Prussian- G blue in colour, only far blacker. In the dry state this tendency to greenness is neutralized by the reddish tinge which the substance sometimes assumes: it was probably used by Newton in the dry state. A mixture of six parts of artificial ultramarineblue, two parts white, and ninety-two parts black, when mingled according to the method of Maxwell's disks, furnishes a colour H quite like that of commercial indigo in the dry state. Cyan-blue. Blue and Violet. FIG. 8.-Fixed Lines and Coloured The next table gives the positions of the coloured spaces in this spectrum, according to the observations of the author: COLOURED SPACES IN THE PRISMATIC SPECTRUM. The space out beyond 0 is occupied by a very dark red, which has a brown or chocolate colour, and outside of the violet beyond 1,000 is a faint greyish colour, which has been called lavender. The third table shows the spaces occupied in the prismatic spectrum by the several colours: In making these observations, matters were arranged so that only a narrow slice of the spectrum presented itself to the observer; thus its hues could be studied in an isolated condition, and the misleading effects of contrast avoided. The figures given in the two latter tables are the mean of from fifteen to twenty observations. The hues of the spectral colours change very considerably with their luminosity; hence for these experiments an illumination was selected such that it was only comfortably bright in the most luminous portions of the spectrum, and this arrangement retained as well as possible afterward. The colours as seen in the spectroscope really succeed each other in the order of their wave-lengths, the red having the greatest wave-length, the violet the least. But the glass prism does this work in a way which is open to criticism; it crowds together some portions of the series of tints more than is demanded by their difference in wave-lengths; other portions it expands, assigning to them more room than they have a right to claim. Thus the red, orange, and yellow spaces are cramped together, while the blue and violet tracts stretch out interminably. Taking all this into consideration, it may be worth while to go one step further, and, without abandoning the use of the spectroscope, replace its prism by a diffraction grating, or plate of glass ruled with very fine, parallel, equidistant lines, such as have been made by the celebrated Nobert, and lately of still superior perfection by Rutherfurd. In Lommel's work, previously referred to, the mode in which a plate of this kind produces colour is explained; at present it is enough to know that the general appearance of the spectacle will be unchanged; the same series of colours, the same fixed lines, will again be recognized; but in this new spectrum all the tints will be arranged in an equable manner with reference to wave-length. According to this new allotment of spaces, the yellow will occupy about the centre of the spectrum, the red and different kinds of orange taking up more room than formerly; the dimensions of the blue and violet will be greatly reduced. Let us suppose, as before, that the spectrum from A to H includes 1,000 parts; then the following table, which is calculated from the observations of Ångström, will indicate the positions of the principal fixed lines: FIXED LINES IN THE NORMAL SPECTRUM. The next table gives the positions of the coloured spaces in the normal spectrum, according to the observations of the author: Fig. 4 shows the normal c spectrum with fixed lines and coloured spaces, corresponding to the tables just given. 20 If these tables are compared with those obtained by the aid of a prism of glass, it will be D seen that the fixed lines and coloured spaces are arranged somewhat differently; the main cause of this difference has already been pointed out. E When, however, we compare the spacing of the colours in the two spectra, it is also to be remembered that it is affected by another circumstance, viz., the distribution of the luminosity in the two spectra does not agree, and this influences, as will be shown in Chapter XII., the appearance of the colours themselves; very luminous red, for example, assuming an orange hue, very dark blue tending to appear G H Orange-red. Orange. Greenish-yellow. and Yellowish-green. Green. Blue-green. Cyan-blue. Blue. Violet-blue. Violet. FIG. 4.-Fixed Lines and Coloured violet, etc. The normal spectrum employed by the author |