THE LINES OE THE SOLAE SPECTEUM. 
153 
sun sinks, there is no real contraction of its several parts, the distance between the fixed 
lines measuring precisely the same at different hours. 
The red sun is often attended with a blue sky ; the diffused light, in fact, always 
exhibits portions of the spectrum which ai*e wanting in the direct rays. This may give 
rise to a singular phenomenon. There is a green glass coloured by means of copper and 
iron which has the power of absorbing the whole of the red and orange rays, but allow- 
ing others to penetrate. If the landscape be viewed through this, all the ordinary 
objects are seen with tolerable distinctness, though of a greenish or bluish colour, but 
anything purely red is extinguished ; and it has happened to Dr. Gladstone, on 
examining the western sky near sunset with such a glass, that he has seen the various 
configurations of the clouds and of the horizon perfectly, whilst the sun itself was so 
httle \isible that its presence would not have been detected, unless it had been specially 
sought for. No black space is produced in this experiment ; for of course that portion 
of the sky immediately between the solar disk and the spectator is sending diffused light 
like the parts adjacent. 
Atmospheric Lines. 
Beyond this absorption of the more refrangible rays, another and more remarkable 
phenomenon presents itself when the sun descends towards the horizon and shines 
through a rapidly increasing depth of air. Certain lines which before were little 
if at all visible, become black and well-defined, and dark bands appear even in what 
w'ere formerly the most luminous parts of the spectrum. This has been observed both 
at sunrise and sunset ; and it is not necessary that the luminary should be just on the 
horizon, or that the absorption of the more refrangible rays should be very complete ; 
indeed these atmospheric lines have been seen when H and k were easily distinguishable, 
and light was perceptible far beyond. Fig. 7 is a map of these lines and bands, com- 
piled from the independent drawings of the two authors, which agree very closely. In 
calling them “ atmospheric,” nothing more is meant to be expressed by the term than 
the mere fact that these lines or bands become much more visible as the sun’s rays pass 
through an increasing amount of atmosphere. Sir David Brewster first observed them, 
and gave a verbal description of them in the paper already referred to *, but his more 
exact and extended observations were made subsequently. In the least refrangible part 
of the spectrum which was first observed by him, the bands X3, YI, Y3, and Z1 become 
very dark as the sun sets, and assume the appearance of broad black bands, supporting 
on each side the finer lines Y and Z. A becomes very wide even when the sun is at a 
considerable altitude, its aspect being that represented in fig. 8, which is on the same 
scale as fig. 3 ; and gradually the lighter portions on each side of the black line become 
quite dark, and the series of lines before it are converted into a dark band, so that it 
presents the appearance of two black spaces divided by a narrow luminous space ; but as 
the sun sets this light also disappears, and it becomes one uniform expanse of darkness. 
The gi’oup a becomes much deeper in shade, but not uniformly so, for the bright spaces 
between the bands are not absorbed. In general the shaded parts between A and B 
* Edinburgh Philosophical Transactions, vol. xii. pp. 529, 530. 
