cf Eye-glafles fir Te/efiopes. 97 
of an oblique pencil as that from colour. I11 both, where 
there is a neceffity for having a large portion of a fphere, we 
have only to make the pencil on fuch lens as (mail as poffible, 
and we may regulate the direction of the rays in each pencil at 
pleafure when they approach the axis of the telefcope. 
To i llu fir ate this, let us compare the effect of the fpherical 
aberration of a lens on an oblique pencil in this pofition with 
that produced by the fame lens, placed as ufual at its focal 
diftance from the image. Let AC, fig. 3. reprefent the femi- 
objeft glafs of a telefcope, CT its axis, E an eye-glafs, and F 
the common focus of both the object- glafs and eye-glafs. Let 
AFC be an oblique pencil of homogeneous rays, G and H the 
■points where the axis and the extreme rays pafs through the 
eye glafs : the aberration of this pencil from the fpherical 
figure of the lens E will be EG 3 - EH 3 ; but as the lens ap- 
proaches towards F, EG and GH, becoming equal, this caufe 
of aberration vanifhes accordingly. The effefts of the lens k 
will be altogether infenfible from the fmallnefs of its aperture ; 
or it might be corrected in the figure of the objeft-glafs, by 
making its aberration negative as much as this is affirmative. 
It has been ufual to conlider that form and pofition of the 
eye-glafles beft that would make the pencils from every part of 
the field interfeft each other in the axis of the telefcope at the 
place of the eye; but this will be found of little confcquence, 
feeing the diameter of a pencil here is generally much lets than 
the pupil, nothing more is requifite than that the eye may take 
in the pencils from the different parts of the field at the fame 
time : but the field of a telefcope will be mod: perfect when 
the conftruftion of the eye-glaflfes is fuch, that the locus ot 
an extreme and of central pencil are at the fame diftance 
Vol. LX XI II. O from 
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