i ~ 6 A Paper to obviate feme Doubts concerning the 
from my eye as the object from the great fpeculum of the tele* 
icope. Upon a pole erefted there 1 viewed the magnified image 
of the half inch, and the alfifhmt marked it by my direction ; 
this being mealured gave the power of the inftrument at once. 
The power thus obtained was corrected by theory, to reduce it 
to what it would be upon infinitely diffant objedls. The 
powers of the reft of the lenfes I deduced from this by a Ca- 
mera-eye-piece, which I made for that purpofe. ABCD (fig. i.) 
reprefents a perpendicular fection of it. The end A fcrewS 
into the telefcope. Upon tire end B may be ferewed any of the 
common lingle-lens eye-pieces. Imn is a fmall oval plane 
fpeculum, adjured to an angle of 45 0 by three ferews, two 
whereof appear at op. When the oblerver looks in at B, he 
may lee the objefil projected upon a Iheet of paper on a table 
placed under the Camera-piece, and meafure its picture a, b, as 
in fig. 2. The power of one lens therefore being known, that 
of the reft was alfo found by comparing the meafures of the 
projected images. 
It may not be amils to mention fome of the advantages and 
inconveniencies attending each of thele methods. When we 
take the focus of an eye- lens, which the firft method requires, 
we are liable to a pretty confiderable uncertainty, and in very 
fmall lenfes it is not to be done at all. Moreover, in calculating 
the power by that focus no account is made of the aberration 
which takes place in all fpecula and lenfes, and increafes the 
image, fo that we rather find out how much the telefcope 
fhould magnify than how much it really does magnify; but in 
determining the power by an experiment we avoid thele diffi- 
culties. 
On the other hand, when the pow T er is very great, the latter 
method becomes inconvenient, both on account of want of 
light 
