of different Species of Glass , 291 
ed through was still decomposed into all its colours, with this 
difference only, that in the spectrum, the colour peculiar to the 
glass or the fluid was more brilliant than the rest. Even the 
coloured flames obtained by burning alcohol, sulphur, &c., seen 
through a prism, do not yield a homogeneous light correspond- 
ing to the colour *. These flames, however, such as that of 
a lamp, particularly that of a candle, and, in general, the light 
produced by the flame of a fire, exhibit between the red and 
yellow of the spectrum a clear and well marked line, which oc- 
cupies the same place in all the spectra. This line will become 
more important in the sequel, and it was one of great utility to me. 
It appears to be formed by rays which are not decomposed by 
the prism, and which consequently are homogeneous. In the 
green space we perceive a similar line, but it is weaker, and less 
distinct, so that it is often very difficult to find. 
It was, however, absolutely necessary for me to have homo- 
geneous light of each colour, and the following was the method 
which I employed. Behind an aperture in a shutter, 1.5 of an 
inch high and 0.07 wide, I placed a prism A (Fig. 3. of Plate 
VII.) of flint-glass, with an angle of about 40°, and at BC, a dis- 
tance of about 13 feet, I placed six lamps, whose light fell 
through narrow apertures on the prism A. The width of these 
apertures was 0.05 of an inch, their height nearly 1.5, and the 
distance of one lamp from another 0.58. The light of the 
lamps which fell on the prism A was refracted by it, and de- 
composed into colours, and afterwards passed through the aper- 
ture in the shutter. From the lamp C, for example, the red 
rays came in the direction of E, and the violet in that of D. From 
the lamp B the red rays passed towards F, and the violet rays 
towards G, &c. At the window of another house, 692 feet 
from A, and at the same height of the plane BAG, I placed 
the theodolite already mentioned, before the telescope of which, 
on t;he horizontal plane, was set the prism H, whose index of 
refraction for the different coloured rays I wished to determine. 
The prism H could only receive from the lamp C the red rays, 
* For an account of various recent experiments on this subject, made by Dr 
Brewster and Mr Herschel, see the Edinburgh Transactions , Vol. IX. p. 433. and 
445.— .Trans i,. 
T 2 
