58 . On the UNEQUAL 
of crown and flint glafs, ought to be one fixth of one fifty-fifth 
part of its aperture. The difference in the focal lengths of the 
eye-glaffes will then render the indiftinétnefs nearly equal in 
the two kinds of object-glaffes with equal magnifying powers, 
in all cafes where their apertures are equal, and their lengths as 
one to fix. 4 
TuerE is, however, a circumftance of the greateft moment 
to be taken into account before this conclufion can be admitted, 
which is, that not merely the diameter of the circle of aberra- 
tion is to be confidered, but alfo the {piffitude-of the rays, both 
within that circle in general, and at different diftances from its 
centre. The rarity of the light in the fimple fpe@rum is fuch, 
that the aberration hurts much lefs than might be expeéled. 
But in the fecondary fpectrum, as two orders of coloured light 
are united, the imperfect union of the rays by the compound 
object-glafs, will hurt the diftin€tnefs’ much more, in propor: 
tion to the extreme divergency. 
On this account, it is to be expected, that the proportional 
lengths of the fpeétrums, when the experiment comes to be pro- 
perly made, will turn out lefs than as one to fix, notwithftand- 
ing the degree in which the diftinétnefs is hurt in the two kinds 
of telefcopes, from the unequal refrangibility of light, rm be 
nearly in that proportion. 
THE principal improvement of refracting telefcopes, initia 
out by the preceding experiments, confifts in an entire removal 
of this aberration from the unequal refrangibility of light. It 
appears from the performance of the {mall telefcope above men- 
tioned, in which the fecondary colour is not removed, that con- 
fiderable advantages may alfo be expected from fubftituting a 
more perfect medium for flint-glafs; from a more perfect cor- 
rection of the aberration from the fpherical figure ; from pre- 
venting that lofs of light by reflection, which takes place when 
light enters into, or emerges from denfe mediums furrounded 
with 
