344 
t»OPULAR SCIENCE REVIEW* 
Fraunhofer, a working optician, of Munich, was the first person 
who attempted this, and they have been called Fraunhofer’s lines. 
In order to do this, he viewed the sun’s light through a prism 
placed in the focus of a small telescope provided with micro- 
meter screws for measurement, and he mapped upwards of 600 
of them, and indicated the most conspicuous by the letters of 
the alphabet. 
Still, this does not explain the meaning of the particular 
lines. The map itself needs interpretation. For this explana- 
tion, and for the mode of experiment required, we are indebted 
to Professor Kirchhoff. The figure shows a diagram of this ar- 
rangement in plan. If we take two wires of any metal, such, 
for instance, as magnesium, and by means of a strong heat, such 
as that of the electric spark, convert a portion of the metal into 
a luminous gas, and place the spark-giver at m opposite the slit 
0 of Fraunhofer’s apparatus, which, in its present improved form. 
is called a spectroscope, we shall see the bright lines characteristic 
of magnesium. Suppose that over one-half of the slit o of the 
spectroscope a small reflector r is placed, as is shown by a front 
view and on a larger scale in fig. 12, and that by means of this 
reflector a beam of the sun’s light s, fig. 11, is reflected into the 
tube, then transmitted first through the lens I, then through 
the prism p, and afterwards through the telescope t, into the 
eye of the observer, and at the same time the electric sparks 
are made to pass between the magnesium wires : two spectra 
will then be seen, one over the other, edge to edge, just as is 
represented in fig. 9. 
By thus comparing the spectra of the different elementary 
liodies with that of the sun, not only was magnesium found to 
be present, intismuch as the bright lines of magnesium coincide 
with certain dark lines in the solar spectrum, but sodium, iron, 
calcium, hydrogen, and eleven other elements — sixteen in all. 
