(^different Species of Glass. 57 
by the rays which go from the eye-glass to the eye. The dia- 
meter of this cylinder varies, and depends on the aperture of the 
object-glass, and the focus of the lenses of the eye-glass. It is 
easy to conceive that this aberration increases with the dia- 
meter of the cylinder. Great care is therefore requisite, in the 
calculation of object-glasses, to attend to the aberration of the 
eye, and to make it disappear from the object-glass. 
If, in the calculation of achromatic object-glasses for the sphe- 
rical aberration, we wish to make this aberration disappear en- 
tirely, the indices of refraction for the flint and crown glass ought 
to belong to the same coloured ray ; for, if these indices belong to 
different rays, the aberration can never be extinguished, notwith- 
standing the most rigorous calculation. As the discovery of the 
lines in the spectrum enables us to determine these points with 
accuracy, they must be considered of great utility in removing 
this aberration. 
Before the discovery of the lines in the spectrum, I determined 
the identity of the refracting powers of two kinds of glass, by 
cementing them together, and forming them into a single prism. 
If the two specula seen by this prism appeared on the same 
place, and without any reciprocal displacement, I concluded that 
their refracting power was the same. After the discovery of 
these lines, however, I found that two pieces of glass might still 
have a different refractive power, without that difference being 
perceived by the above method. This difference in refracting 
power was not only found in pieces of glass taken from different 
parts of the same crucible, but even in pieces taken from the two 
extremities of opposite sides of the same piece of glass. By re- 
peated experiments on the manufacture of flint and crown glass, 
I have succeeded to such a degree, that, in a crucible containing 
400 lb. of flint-glass, two pieces, one of which is taken from the 
bottom, and the other from the top, have the same refractive 
power. 
In observing the great quantity of lines in the solar spectrum, 
we might be led to believe that the inflexion of light at the nar- 
row aperture in the window-shutter had some connection with 
them, though the experiments described do not give the least 
proof of this, and indeed establish the contrary opinion. In 
order to put this beyond a doubt, and also to make some other 
observations, I varied the experiments in the following manner : 
