IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 801 
Spectrum to a “colour-blind” eye, as being near the place of the Fraunhofer 
line F, or the “blue,” z.¢. the blue-green or glaucous, Hydrogen locality. 
Now if blue be seen when under some mixture with other colours which 
may give it a cold, dark, inky hue,—how all thorough artists do detest it! 
How many a natural scene has been condemned esthetically for appearing to 
them, with their tri-chroic, uncoloured, eyes, lugubrious; weighed down by that 
heavy and most forbidding tint. Yet a di-chroic eye has seen the same identical 
region at the very same time, luminous, brilliant, glowing with red, and not 
without sufficient browns, purples, violets and even points both of black and 
white, to give force of character, variety of effect, and make its owner happy 
for the day. 
In short, if there is any colour which may be on the whole, though it is 
certainly not in every particular, somewhat defective in the colour sensation of 
powerfully Di-chroicised (we can no longer call them colour-blind) individuals, — 
it is not red, but yellow; or that colour which, in painting, being added too 
largely to almost any mixtures of red and violet, makes them dirty, messy, 
odious at once. The reason too of so much freedom for di-chroite eyes, from 
the world’s vulgarising leaven of overpowering yellow, is plain enough from our 
previous spectral examinations of colours. For the very essence of a di-chroic, 
as contrasted with a mono-chroic, medium is there shown to be, the dulling out 
of the yellow, or middle, region of the spectrum. So that whether eyes are 
tinged with visual purple, gray, green, coffee-brown, or anything else, so long as 
the medium is spectrally di-chroic, the yellow, and more especially the yellow- 
green or citron components in the scene must be shut out, to a greater or less 
degree. 
But before that citron region, usually by far the brightest in our natural-eye 
spectrum, (indeed too bright, generally, for good vision of both the red and the 
violet simultaneously with it), is completely excluded, by increasing depths of 
di-chroicising media,—strange effects may be produced by its partially darkened 
tints mixing with others. Thus—take the particular red cloth cover of Dr 
GEORGE WILson’s symbolically bound volume. On placing it in sunshine, and 
looking at it through a dark (as jth to a ;4th)* solution of Judson’s green 
dye,—the effect is, that it is seen of a brighter red. 
But if we next look at it through a pale (s$oth) solution of the same dye, 
a something so weakly green that it produces little or no effect either on 
bright green grass or things in general,—what do we see ? 
A black book! Noi very densely or darkly black, but nearer black than 
* From gz to z's, according to the brightness of the sunlight at the time, is the strength of 
solution which has most effect in turning the greenest grass into the most brilliant scarlet. Stronger 
solutions as jy and 75 both stop too much light, and make green grass only dully orange or buff. 
