
IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 789 
Again for Telescopic utilisations, as for ensuring the finest definition to the 
Solar limb,—the most strictly monochroic, though dark, media may be selected 
from our Appendices, say Ruby-red glass, Judson’s crimson dye, or ammonia- 
chloride of copper, blue; where the deep colour will ensure their ‘acting as sun- 
shades as well. But, for the purpose of practically achromatising, a simple 
lens, with the least possible loss of light, bichromate of potash solution appears 
to excel every other tint tried as yet, being both a monochroic, and in nearly 
the most luminous part of the spectrum. 
NOMENCLATURE OF COLOURS, OPTICALLY. 
For Double-Star observers chiefly. (See Plate 3, No. 43 of this Vol.) 
If all the spectral colours discerned in practice must, for theoretical purposes 
be reduced to a primary three only; then those three are, according to what 
has for some years past been generally held, and is now given by my renewed 
observations, taken under favourable circumstances, not red, yellow and blue; 
nor even red, green and violet, but rather 
RED, 
CITRON, 2.é. yellow-green, and 
VIOLET. 
Red, therefore is characteristic of the red-end, or beginning of the spectrum; 
Citron, of the spectrum’s middle and most luminous region; and Violet, of the 
spectrum’s latter, closing, and more refrangible end. 
But in practical colour-naming, it is both more convenient to start from a 
basis of a greater number of separate, if nature-presented, tints; and, over 
and above what may be done after an approximate fashion, by rude mixtures 
of the primary three,—there are, and with all exactness and certainty, very 
many more colours distinctly discernible in the simple solar spectrum as 
separate, peculiar, existences, when powerful practical methods have been 
correctly applied to show the colours on an extended scale, say 50 feet long, 
and in their utmost possible spectral purity, or freedom from admixture of one 
with the other, or with anything else.* Yet, after all of that increase in number 
of visible colours shall have been accomplished, a single solar spectrum, how- 
ever rich in many, by no means gives us all, of Nature’s colours as we see them 
day by day. Some, for instance of botanical colours,—and often the most 
exquisite of them all,—are only to be imitated spectrally by taking two spectra, 
and making their opposite ends overlap. While there are other tints still, which 
are only spectrally re-producible by three different spectra, or three portions, 
not naturally adjacent, of one spectrum, being artificially combined together, as 
in Professor CLERK MAXWELL’s most ingenious and instructive “ Colour-box.” 
* See forward to pp. 791 and 792. 
