qf different Species of Glass. 35 
which of them the aberration produced by unequal refrangibili- 
ty is the best compensated, if we cover one-half of each of 
them by a screen passing through the centre of the object-glass 
in a straight line. In those where the margin of a distant ob- 
ject is most distinct, the aberration is best compensated. In 
making this' comparison, we must attend only to the distinctness 
of the object, and not be deceived by the colours, because one 
object-glass may shew less colour than another, and yet its 
distinctness be less. This detailed method of finding the best 
ratio of dispersion, is useful only for determining how much the 
aberration of the faintest rays ought to exceed that of the bright- 
est rays. This result will be still more accurate if it is obtained 
by trials made with greater object-glasses, whose apertures are 
in the ratio of the greatest possible focal length. It is scarcely 
necessary to add, that the aberration of sphericity was corrected 
in all the object-glasses employed in these experiments. There 
is still another aberration which takes place in the eye itself, 
and to which we ought to pay attention, if we wish to find the 
best ratio of the colorific dispersion. 
In placing the red colour of the spectrum in the middle of the 
field of the telescope of the theodolite, and in adjusting the eye- 
glass so as to be able to distinguish the fine micrometer wires, 
these wires will be no longer seen when the violet rays enter the 
field, the eye-glass remaining fixed. In order to see the wire in 
this colour, we must bring the eye-glass much nearer the wire ; 
that is, more than double the aberration produced by the un- 
equal refrangibility of the two kinds of rays in the eye-glass. 
This proves that the difierently coloured rays in the eye have 
not the same focal distance, and that the eye is not achromatic 
The distance to which the eye-glass ought to be displaced in 
different colours, in order to see the wire distinctly, enables us 
to calculate this aberration in the eye, which is by no means 
small ; but we must take into account the aberration produced by 
the eye-glass itself It is scarcely necessary to state, that, in 
* The want of achromatism in the human eye has been long ago pointed out 
by Dr Maskelyne, Dr Blair, and Dr Young. The last of these philosophers con- 
cluded, that the dispersive power of the eye collectively, is one-third of the disper- 
sive power of crown-glass, at an equal angle of deviation. Mr Ramsden maintained 
the extraordinary opinion, that the separation of coloured rays is only observed 
