[ 5 61 ] 
undergo in pafling thro’ the telefcope, will no way 
alter the pofition of the images which the rays, that 
have palled thro’ the fegments, are tending to : For, 
as has been already obferv’d, a number of reflexions 
and refractions may repeat the images, and alter their 
magnitudes ; but can make no alteration in their pro- 
portions. 
Therefore this way of fixing the divided glafs to a 
reflecting telefcope, which was the third method pro- 
pos'd, is, by far, the belt ; as luch telefcopes of mo- 
derate and manageable lengths, when well made, are 
capable of magnifying confiderably, and Ihewing ob- 
jects to great advantage. This micrometer’s being 
applicable to the reflecting telefcope, with fo much 
certainty, is no inconfiderable advantage : For any 
one will eafily underltand, that, to mealure the dia- 
meter of a planet exaCtly, it is neceflary, that the pla- 
net be magnify’d, and Ihew’d diftinCtly, which could 
not be obtain’d, in the common way, without very 
great lengths ; fuch as render’d it very difficult, not to 
fay impracticable, to take exaCt meafures. Belides, 
the common micrometer is limited, in this refpeCt, 
upon another account ; viz. becaufe the diameter of 
the planet cannot be meafur’d, without having the 
whole planet within the field of the telefcope which 
confines the magnifying power within very narrow 
bounds ; whereas, by this method, nothing more is 
requir’d, than to fee the contaft of the edges, which 
allows the magnifying power to be increas’d at plea- 
fure. 
In the common micrometer, the objeCt is to be 
taken between two wires, fo that the contad of its 
edges with thofe wires cannot be obferv’d at one view ; 
4 B and 
