
IN PRACTICAT ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 793 
VISION OF COLOURS, THROUGH COLOURED MEDIA. 
So long as we confine ourselves, in such vision, to media of a MONO-CHROIC 
nature, very little, if any, unusual or particularly interesting, effect will be ob- 
served. A general glare of red, will no doubt appear in all the lights of a 
landscape, and especially of the sky, when we look through the ruby-red glass 
of railway signal-lamps ; or of b/we, through solutions of ammonia chloride of 
copper ; or of yellow, through Bichromate of Potash. But neither the indigen- 
ous and previously existing red, nor the blue, nor the yellow substances of the 
earthly scene are themselves much discriminated, intensified, or beautified 
thereby. . 
The moment, however, that we employ DI-cHRoIC media, and of any colour, 
some most startling changes may be witnessed. 
Take for instance a weak, and therefore to the world at large, a slatey-blue 
solution of Oxalate of Chromium and Potash and look through it at the reds, 
whether of flowers or brick-fields in the landscape. In place of their being 
dulled, as they would infallibly be if mixed up with so much impure blue as 
colours on an artist’s palette,—they now start forth brilliantly, gorgeously, red ; 
perhaps even three times more red than they ever were before !* 
Or take a more unpromising medium still, a dark-blue and black-green 
(Judson’s Aniline dye), and let it be as deep almost as writing ink. What 
should we expect to see through such an apparent hindrance to any vision at 
all, as that ? 
The following is what I did see on the first occasion of trying the experi- 
ment, and you may judge of my surprise. 
The chief feature already in the scene was a white cottage with a blue- 
slated roof, in a broad, sun-illumined field of bright green grass, with some red 
brick buildings in the distance. I then put the flat black bottle of Judson’s 
most dark and blue-green fluid before my eye,—and lo! the view was still of 
day-light brightness, there were still the red buildings in place, still the brilliant 
white cottage with its distinctly blue-slated roof,—but it was now standing in 
a field, not of green at all, but of glaring and evident ver milion ! 
I next looked down, through the dark medium as before, into a neighbour’s 
garden just renovated for the approaching spring. There was a rockery of 
various coloured stones, leafless tree trunks of warm dark tints, and a brilliant 
gravel serpentine walk,—but it seemed to be bordered now, not with green 
* The Oxalate of Chromium and Potash was well examined by Sir David Brewster in his best 
days ; and he distinctly observed that though bluish in weak solutions, it allowed only red light to 
pass through its thicker solutions. Our recently observed effect depends upon that, but is accompanied 
by something further. 
