56 On the UNEQUAL 



other. I 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 (till appeared of fuch a breadth as rnuft 

 necefTarily hurt the diftinctnefs. 



I HAVEjiere 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 indirect trial, that the aberration from unequal refrangibi- 

 lity would not differ very materially in thefe object-glafTes, 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 indiftinctnefs proceeding from it ought to be 

 the fame in both object-glades, when the apertures and magni- 

 fying powers applied, are as the fquare roots of their refpective 

 focal lengths. 



It would appear that the aperture of an object-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 

 fcarcely to exceed two inches. 



From 



