70 On the UNEQUAL 
Tue application of the demontftrations of HucEnivs to the 
correction of the aberration from the {pherical figures of lenfes, 
whether folid or fluid, is then taken notice of, as being the next 
ftep towards perfecting the theory of telefcopes. 
Next it appears from trials made with objeét-glaffes of very 
large apertures, in which both aberrations are corre¢ted as far . 
as the principles will admit, that the correction of colour which 
is obtained by the common combination of two mediums 
which differ in difperfive power, is not complete. The homo- 
geneal green rays emerge moft refracted, next to thefe the uni- | 
ted blue and yellow, then the indigo and orange united, and 
laftly the united violet and red, which are leaft refracted. 
Ir this production of colour were conftant, and the length of 
the fecondary fpectrum were the fame in all combinations of 
mediums when the whole refraction of the pencil is equal, the 
perfect corre€tion of the aberration from difference of refrangi- 
bility would be impoflible, and would remain an infurmount- 
able obftacle to the improvement of dioptrical inftruments. 
‘Tue object of the next experiments is, therefore, to fearch, 
whether nature affords mediums which differ in the degree in 
which they difperfe the rays compofing the prifmatic {pectrum, 
and at the fame time feparate the feveral orders of rays in the 
fame proportion. For if fuch could be found, the above men- 
tioned fecondary fpeétrum would vanifh, and the aberration 
from difference of refrangibility might be removed. The re- 
fult of this inveftigation was unfuccefsful with refpect to its 
principal obje&t. In every combination that was tried, the 
fame kind of uncorrected colour was obferved, and it was 
thence concluded, that there was no direct method of remo- 
ving the aberration. 
Bur it appeared in the courfe of the experiments, that the 
breadth of the fecondary {peétrum was lefs in fome combina- 
tions than in others, and thence an indirect way opened, lead- 
ing to the corre¢tion fought after; namely, by forming a com- 
pound 
