r m ] 
Axis, fhall be colleded by means of the Lens exaftly 
in another Point, its Aperture may then be extended 
to its furtheft Limits ; and that is, till the whole 
Pupil of the Eye (or the whole Portion of the Eye- 
glafs to be ufed, when that becomes neceifarily lefs 
than the Pupil) be filled with Rays proceeding from 
the Speculum, and flowing from one Point of the 
Objed, but no farther 5 becaufe this is a Limitation 
made by Nature in the Strudure of the Eye itfelf : 
And in Telefcopes whofe Conftrudion is fuch as we 
have now defcribed, the largeft Aperture of the 
Speculum that can ever be of Ufe, will be to the 
Diameter of the Pupil of the Eye, very nearly, in a 
Ratio compounded of the Ratios of the Focal 
Length of the Speculum to the Diftance of that 
focus from the Lens, and of the Diftance of the 
Lens from the Focus of the Telcfcope, to Unity ; 
That is, of B B to BH) and of RH to i j which 
Proportion holds, whatever be the Charge or the 
Power of Magnifying. 
But if Inquiry be made as to the Charge moft 
proper and convenient, that will be determined beft; 
by Experience, in thefe, as well as in all other forts 
of Telefcopes : However, on Suppofition that one of 
a given Length has its Aperture and Charge rightly 
ordered and proportioned, the Rule for preferving 
the fame Degree of Brightnefs and Diftindnefs, in all 
others of a like Conftrudion, will be, to make the 
Apertures, and magnifying Powers, diredly as the 
Focal Lengths of the Speculums j which fhews the 
vaft Advantage and Perfedion of thefe Telefcopes, 
above the common refleding ones ; where, accord- 
ing to Sir Ifaac Newtons Rule, the Apertures, and 
U u 2 Powers 
