
IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 795 
colour experiment is, that the thing looked at, shall be brilliantly illuminated, 
while the thing looked through, shall be comparatively in the shade, as well as 
physically darker: for, if these conditions be well secured, the blue of the blue- 
green in the dark bottle looked through, so combines its undulations with those 
of the blue of the thing looked at, that they make something approaching to 
white light, in place of double blue-dark. 
Hence if thus through the, in itself most dark,* bottle, we contemplate a 
piece of known cobalt-blue glass held before us in the sun-shine,—its blue 
colour is for the time entirely gone and nothing left behind except a faint 
residual pinkish tinge, in an otherwise transparent and white glass plate. 
In the same manner, looking at another, and weaker bottle of Judson’s green, 
but extra well illuminated,} its green is knocked out of it, and you might fancy 
the contents little but pure water. 
CHEMICAL INDICATIONS, BY DI-CHROIC SPECTRAL MEDIA. 
While green of grass is turned so positively into glaring red, there are other 
green substances which resist, and some which even change in the opposite 
direction,—under the influence of the di-chroic blue-green black bottle. 
Thus while all plant leaves that I have tried, became more or less red,— 
litmus paper and litmus dyed cloth, grandly red ; and two samples of green 
paper-hangings became decidedly red ;—yet on the other hand,— 
Blue of heaven, 
Blue of slates, 
Blue of sulphate of copper, 
Blue-green of acetate of copper, 
Certain green-painted Venetian blinds, and 
Nine samples of green paper-hangings 
refused to become red at all, and some of them verged towards glaucous. 
* One of Judson’s } ounce bottles of concentrated dye, mixed in 20 ounces of hot water and 
allowed to cool, gives nearly the requisite strength and darkness, in a flat bottle one inch thick 
internally. On trying solutions of various strength in an angular or prism-shaped bottle the disper- 
sion was not found to be anomalous as with fuchsine, but normal as with glass, though with the 
addition of looking through the glass-prism with a flat film of Judson’s green interposed, and thereby 
reducing the whole continuous spectrum of daylight to a red band and a green band, with a black 
band between them. 
+ As the parallel, though opposite, example in spectroscopy of the importance of the intense 
illumination of the thing looked at, than the thing looked through, to bring about the interference of 
waves and inversion of the simple appearance of the light of the nearer substance ;—if we hold a spirit 
lamp burning salt between a spectroscope, and a dull distant object giving a weak day-light spectrum, 
no inversion takes place ; and the salt lines of the lamp appear bright, utterly hiding thereby the 
dark, but only grey, D lines of the clouds. 
But if we hold the same spirit lamp between the same spectroscope and the Sun, the bright salt 
lines of the poor little lamp are inverted instantly and add black, not white, to the already very dark 
Solar D lines. 
VOL. XXVIII. PART III. 9Y 
