2 ON THE ACTION. OF TRANSPARENT BODIES: 
to have a greater refractive power than the latter. If we again 
take other two prisms of the same substances, having such re-. 
fracting angles, that a ray of light is equally refracted by both, 
and if we examine the spectrum which each of’ them affords, 
by admitting the sun’s light into a dark chamber, it will be 
found that the spectrum formed by the flint-glass is much 
longer than the spectrum formed by the rock-crystal. The 
flint-glass is therefore said to. have a greater dispersive power 
than the rock-crystal, or a greater power of separating the ex- 
treme rays from the mean ray of the spectrum. If we then 
take other two prisms with their refracting angles of such a 
magnitude that they produce spectra of said lengths, it will 
be found by a particular mode of examination, that the colour- 
ed spaces have not the same size in the two spectra. The red 
and green rays will occupy more space, and the blue and vio- 
let ones less space, in the spectrum formed by the rock-crystal, 
than in the spectrum formed by the flint-glass. This want of 
proportionality, or irrationality in the coloured spaces of differ- 
ent spectra, is not of such a magnitude as to be visible upon a 
mere examination of the spectra themselves. In order to ob- 
serve it, we must make the prism of flint-glass refract in op- 
position to the prism of rock-crystal; and if we look at the 
bars of a window through the combined prisms, we shall per- 
ceive on the side of the bar to which the vertex of the flint- 
glass prism is directed, a fringe of green light, and on the 
other side of the bar, a purple or wine-coloured fringe. The 
flint-glass, therefore, acts less powerfully upon the green rays 
than the rock-crystal, or they are less separated from the red 
extremity of the spectrum. These uncorrected colours, have 
been called the secondary spectrum, and form the principal ob- 
stacle to the perfection of the achromatic telescope. 
In, 
