Dl?. - miller’s EXETER LECTURE. 
343 
temperature of the sodium and its light must be considerably 
higher than those of the glowing body behind it. What is true 
of the vapour of sodium is true also of other vapours. 
The light of the sun affords us a remarkable instance of a 
case in which the first condition is realised. The spectrum of 
the sun’s light is not continuous, but is crossed by a multitude 
of fine black lines, a few of which are represented in fig. 1 on 
the Plate. These lines, you will observe, vary in number, in 
blackness, and in definiteness in different parts of the spectrum. 
A facsimile of the lines in the neighbourhood of the line 
marked Gr as photographed by Mr. Rutherfurd, is shown in fig. 
10, upon a scale eight times larger than the spectra shown in 
the coloured plate. 
Fig. 10. 
The fact of the existence of these lines was first noticed 
between sixty and seventy years ago, by Dr. Wollaston, and 
anyone may easily observe a few of the principal lines by pro- 
ceeding as he did ; placing himself in a darkened room, allow- 
ing a beam of daylight to come in through a chink of about a 
twentieth of an inch wide, like that formed by the edge of a 
nearly closed door, and then at a distance of 10ft. or 12ft. view- 
ing this line of light through a glass prism held close to tlie 
eye, with its edge parallel to the line of light. Little, however, 
was it imagined when these lines were first seen that in them 
lay the means of ascertaining the chemical components of the 
sun. Many among you, from what I have already said, will, 
however, see how this knowledge is obtainable. 
The sun itself is not a mere globe of glowing iron. It con- 
sists of a central, intensely heated nucleus, above which is an 
atmosphere filled apparently with white-hot solid particles dis- 
tributed in the form of vast clouds over the whole surface, and 
outside this powerfully luminous atmosphere is another less 
heated gaseous stratum containing the vapours of a variet}^ of 
bodies, most of them metallic in their nature. 
The black lines which we see in the solar spectrum are the 
effects produced b}’^ these cooler but still intensely heated 
metallic vapours, upon the light emitted by the cloud -like 
luminous surface of the sun. 
How are we to learn what the bodies are in the sun by which 
these black lines are formed ? The first thing to be done is to 
measure their position accurately, and to make a map of them. 
