of Edinburgh , Session 1881-82. 
363 
and gaseous, which exist on our planet exist also in many of 
these bodies. It can also indicate, that some of these far distant 
suns are moving through space in certain directions, and what is 
their rate of motion. 
Our own sun, as the source to us of heat and light, is naturally 
and properly the body to which this wonderful instrument has 
been most directed ; with the view of detecting the different sub- 
stances which enter into its structure, as well as the density, state 
of aggregation, &c., in which they occur, and the rates at which 
they are moving. 
The result of these spectroscope researches has been to discover 
the nature of many of these substances ; indications of them being 
found in the rays of the sun, — when these rays are so divided and 
expanded by prisms as to show the well-known different colours 
extending from red to violet. Among these colours, about eighty 
years ago, certain black lines, i. e . , lines devoid of light, were 
discovered, which long puzzled philosophers. 
Though our own Brewster in the year 1841 conjectured a pos» 
sible explanation, which was afterwards found correct, I understand 
that the true discovery of the cause of these black lines is due to 
Professor Stokes and to the German philosopher Kirchhoff. They 
explained that the black lines were due to the absorption or reten- 
tion in the sun’s atmosphere of certain definite rays of light coming 
from the incandescent surface of the sun below its atmosphere. 
By a set of ingenious chemical experiments, which it would be 
irrelevant at present to explain, even were it possible for me to 
do so, it was ascertained what these substances are whose light 
is so interrupted and absorbed by the sun’s atmosphere. 
It is sufficient for me to say that astronomers and chemists are 
agreed as to the nature of many of the substances ; and accordingly 
we find in the table appended to Professor Piazzi Smyth’s memoir 
the names of these substances as indicated by the black lines in the 
solar spectrum. 
Before this table was constructed, the best one available was that 
of Professor Angstrom of U psala. But that table was defective in 
several respects, as Professor Piazzi Smyth points out. Some of the 
most important black lines, at each end of the prismatic scale, are 
a wanting in it. Professor Smyth has supplied these blanks, and has 
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