48 THE MICROSCOPICAL NEWS. 
of solar prominences, and very different to that of ordinary day~ 
light. You will notice they are reversed—dark, with bright lines, 
and that all the bright lines correspond in position to the dark lines 
of the solar spectrum. 
The next diagram will show you the spectrum produced when 
the rays from a fixed star are passed through a prism of glass. 
There is the coloured spectrum, containing dark lines too, but they 
take different positions to those I have shown you in the solar 
spectrum. 
So is it also with other stars, the spectrum of Aldebaran, a star 
of the first magnitude in the eye of the constellation Taurus, is 
now before you, with Orionis also. There is the coloured band, 
and many dark lines which occupy the same relative positions as 
many of the solar lines. 
It is not my intention to treat of the nature of these lines, but 
my reason for introducing the subject is to explain that, though 
they vary with different sources of light; that taking the solar 
spectrum these lines occupy constant positions, ceteris paribus, and 
the most prominent of them are used in optical work to identify 
some particular portion of the spectrum, or some of the effects on 
a ray of light by its passage through different prisms. 
We may now see upon the screen a much enlarged figure of the 
solar spectrum ; the A line is in the extreme red, B in the light 
red, C in the orange, D in the yellow, E in the green, F in the 
blue, G in the indigo, and H in the violet. 
The relative separation of these lines from each other is called 
the dispersion of the ray, and it should be carefully studied, as it 
has an important bearing upon the construction of lenses, and also 
upon the manipulation of immersion objectives. 
If we take two prisms, the one of crown glass and the other of 
flint, we shall find that if they both possess the same refracting 
angle, say 60°, the flint prism will deflect the violet rays to the 
greatest extent, and if we alter the refracting angle of the flint to 52° 
we shall get spectra, in which the D line is equally refracted in both 
cases ; but the flint, even then, gives a spectrum of nearly double 
the length of that yielded by the crown glass, and if we wish to 
obtain a band of colour of equal length with that produced by a 
crown prism of 60°, we shall have to make the flint with 30° re- 
fracting angle. This will bring the B lines and the H lines nearly 
coincident, but a closer inspection will show us that the inter- 
mediate lines do not coincide,—a fact of vital importance to the 
practical optician. 
A flint prism of 30° will thus correct the dispersion of a crown 
prism of 60°, that is to say, there will be no colouration of the ray, 
but it will be deflected from its rectilinear path, as you will see in 
the diagram. 
