IN PRACTICAL ASTRONOMY, SPECTROSCOPICALLY EXAMINED. 785 
And the last 5 have transmitted three or more bands, so as to admit the 
name of 
POLyY-CHROIC. 
When the media are very faint and thin, they will, indeed, all of them 
transmit something of every part and all the colours of the continuous spectrum 
of a gas light. But for our present inquiry, such action would still be termed 
MONO-cHROIC. Impure mono-chroic no doubt; but that, much rather than 
Poly-chroic, because the several colours which they do so transmit are all con- 
nected, one with the other, and form all together a single symmetrical great 
wave of light. 
When however the media increase in depth of colouring matter, the trans- 
mitted light narrows in spectral place, and at length either makes one band 
only, as a decided and pure MONO-cHROIC; or two distinctly and actually 
separated-by-darkness bands, as a DI-CHROIC; or more of such discontinuous 
bands of spectral light, as a POLY-CHROIC. 
With still greater depths of the media, one of the two bands of a DI-cHROIC 
sometimes gets an advantage over the other, until at length one of them is 
entirely lost; and the other alone remaining, the medium becomes a MONO- 
CHROIC again, but of a higher and purer order than at first. Of an otherwise 
most noteworthy order too, for the band left outstanding at the very last, is 
usually the one least characteristic of the colour of the fluid as generally known 
to the world: and the fact has been already remarked by some investigators 
long ago. to the extent of noting that some bluish substances did, when in in- 
- creased thickness of layers, transmit red light only. (See also note to p. 784.) 
Finally the poLy-cHroic media. proper, though approaching the phenomena 
of the Dz-chroices, are never such clear and positive colours as either MONO- 
CHROICS or Di-CHROICS; and rather serve the purpose of shades for eventually - 
extinguishing all light and colour. 
Hence my first attempt at arranging colour-media according to these features 
of their spectral behaviour, runs thus :— 
MONO-CHROICS. 
Ammonia Chloride of Copper. 
Ruby-red glass. 
Judson’s Marone red (Aniline series). 
Magenta red. 
Crimson red. 
Port, Claret and other wines. 
Iodine tincture. 
Yellow-brown glass. 
Bichromate of Potash (Potassium). 


