814 PROFESSOR PIAZZI SMYTH ON COLOUR, 
QUALITIES OF SIMPLE GLASSES—continued. 
Rusy-REp, is eminent, when thick, as a transmitter of one, but rather broad, band of light 
only ; transmitting that one too, easily, while stopping all others entirely, and the place of the 
band transmitted (when through five ordinary ruby-red glasses, and from a given intensity of 
light origin) is right on the place of the Solar C line. 
Therefore this glass may perhaps be utilised in somewhat facilitating the observations of 
hydrogen manifestations over the surface of the Sun. 
It is also a great corrector of want of achromaticity in telescopic images, but is not very 
agreeable to the eye. 
CoBALT-BLUE, is in many points the very opposite of Ruby-red, and yet transmits red, and 
even redder light, than the latter. 
It is opposite to it in this, that it (cobalt-blue) transmits not one beam of light only, but 
patches of multitudes all along the spectrum ; and chiefly at either end of it: so that, when 
used for a sun-shade glass, it positively sets off at the worst any want of achromaticity of a 
telescopic image. 
The colours which cobalt-blue most antagonises, and cuts up into narrow bands, are, Ist, 
orange-red, and then green-yellow. 
The colours it most transmits are ultra red (between great A and little a, Solar), and then 
blue light (between F and G). 
Therefore this glass 1s to be used for the faint ultra red of the spectrum ; no other sort of red- 
transmitting glass, showing red so far to the ultra red end. But its range of red is very limited, 
and it is even peculiarly imimical to the Solar C line and its neighbourhood. 
YELLOW-Brown, admits a rather broad beam of light on the red side of D, transmits that 
well, and stops others powerfully. 
It is simply useful for anti-actinie purposes in photography. 
GREEN GLASS, does for the green side, what yellow-brown glass does for the red side, of the 
line D. It transmits rather a broad band of greenish light, transmits it well, and stops all 
others more and more powerfully, in proportion as removed from it in spectrum place. Hence 
green colour, even when slight, as in “heavy lead flint glass” for prisms, though transmitting 
green, of course, and even blue, antagonises all the violet, lavender, and gray. Hence for these 
parts of the spectrum, whether Solar, or chemical, whether to see the H lines of the Sun, or 
the violet lines of carbo-hydrogen flame, prisms should be selected of white flint glass only. 
But the green of “heavy lead glass,” differs remarkably touching red light from the green, 
(stained-green) glass plates above described, for it transmits much red light freely ; and hence 
prisms made of that heavy lead glass, with its very powerful dispersion qualities, may be 
employed to much advantage, in spite of their greenness, on the Red end of the spectrum. 
COMBINATIONS OF COLOURED GLASSES. 
No combinations of any of the above coloured glasses, transmitted light,— 
(1.) Either so far towards and into the ultra red as cobalt-blue glass ; though that also 
transmits much green, blue, and violet light. ; 
(2.) Or so nearly monochromatic, and easily or intensely, as well as so exactly over the C 
line, as ruby-red glass ; though its band is rather a broad one. 
(3.) Or so far towards the violet as blue-lilac ; though that transmits red light as well. 

