784 PROFESSOR PIAZZI SMYTH ON COLOUR, 
“(3.) In every case the colour of glass or crystal corresponds with the 
portion of the spectrum most freely transmitted.” 
Now each of these supposed laws of coloured glasses and spectral colours, 
is the subject of frequent contradiction by our observations ; thus,— 
1’. Red light is spectrally transmitted most red by media not generally 
esteemed red at all; but held rather to be characteristic of, to abound in, and 
transmit most freely, rays at the blue end of the spectrum. 
2’. Cobalt-blue glass instead of absorbing as asserted all the red rays, trans- 
mits some of them most vividly. And this has been long known and exten- 
sively used in optical experiments for certain purposes by many scientists,— 
though somehow or other passed over in 1877 by the particular text-book 
quoted ; and therefore requiring to be re-asserted very distinctly. 
3’. In multitudes of cases the colour of some glass or crystal according to 
usual acceptation, instead of being the same as, is the very opposite of, that 
part of the spectrum most freely transmitted by them. Thus a certain dark 
blue-green dye, of which we shall have to describe some still more striking 
habitudes presently, transmits under certain conditions only one spectral ray of 
any kind of light, and that one is of a burning, fiery red.* 
Of course there is an explanation of these apparent anomalies. But the 
anomalies are so numerous, and of such wide practical bearing, as to form in 
themselves a law of Nature, which cannot be safely neglected in any system of 
School Philosophy. And if, out of 29 colouring substances spectrally examined 
here, 12 of them have been accordant with the usual law, another 12 have 
been positively against it, and 5 have been of a more difficult character still. 
PHYSICAL, As acainst OPTICAL, 
ARRANGEMENT OF COLOUR MEDIA. 
Each of the three classes of spectrum-modifying substances just alluded to, 
contains examples of most various and diverse tints according to eye-observa- 
tion. They cannot therefore, for their physical qualities, be efficiently classified 
by visible colour at all; but must be rated rather according to the spectral 
action itself just alluded to, and whose effects may be summarised thus :— 
The first 12 substances have transmitted in the spectrum, for all depths ot 
their respective tints, only one band of light, and may therefore be called 
MONO-CHROIC. 
The second 12 have transmitted, in all their deeper tints, two widely distinct 
spectrum bands, and are therefore to be called 
DI-CHROIC. 
* This fluid, as being one of the aniline series of coal-tar dyes, must be of too modern discovery 
or manufacture to have come under the piercing examination of the late Sir David Brewster; though 
he had found other coloured solutions having in different, but lesser, degrees the property just men- 
tioned ; viz., that with increased thicknesses they transmitted not bluish, but red light only. 
“ised Cee (ain é. 
