56 On the UNEQUAL 
other. 1 farther reduced the aperture of the fhort one to one 
inch, when it became manifeftly clearer than the long one, 
though, upon examining the coloured fringes, by covering half 
the object-glafs, they ftill appeared of fuch a breadth as muft 
neceflarily hurt the diftinétnefs, 
I uave.here given the refult of this experiment as I find it 
noted down. Being made with no view to the determination 
of the point in queftion, the accuracy neceflary for that purpofe 
was not obferved. It would appear, however, from this grofs 
and indireét trial, that the aberration from unequal refrangibi- 
lity would not differ very materially im thefe object-glafles, fup- 
pofing their apertures and focal diftances to be equal; though 
in one the partial correction of colour is effected by a combina-~ 
tion of flint-glafs and crown-glafs, and in the other by a com- 
bination of crown-glafs and fpirit of wine, with an effential 
oil. If this aberration were exactly equal in both combina- 
tions, the mifty indiftinétnefs proceeding from it ought to be 
the fame in both objeét-glaffes, when the apertures and magni- 
fying powers applied, are as the fquare roots of their refpective 
focal lengths. : 
Ir would appear that the aperture of an ohiee. glafs, com- 
pofed of crown and flint-glafs of thirty-two inches in focal 
length, ought not to exceed two inches, and therefore that three 
inches is too large an aperture for one of forty-two inches focal 
length ; for the lengths in thefe two cafes ought to be as four 
to nine. In fome telefcopes of this latter kind, I have obferved 
a great deal of uncorrected colour, which prevents them from 
bearing magnifying powers, in proportion to the aperture of 
the object-glafs. “It is indeed but feldom that the union of the 
differently refrangible rays is fo perfect as the conftruction ad- 
mits. I have met with others in which the real aperture is fo 
far contracted, by diaphragms placed within the tube, as. 
f{carcely to exceed two inches, 
From 
