( H7 ) 
I. The Defcription of a new Injlrument for taking 
Angles. <By John Hadley, Efqy Vtce-Tr. 
(f. S. communicated to the Society on May 1 3 . 
173K 
T HE Inflrument is defign’d to be of Ufe, where 
the Motion of the ObjeCts, or any Circum- 
ftance occafioning an Unfteadinefs in the common In- 
flruments, renders the Obfervations difficult or uncer- 
tain. 
The Contrivance of it is founded on this obvious 
Principle in Catoptricks : That if the Rays-of Light 
diverging from, or converging to any Point, be re- 
fiedled by a plane polifh’d Surface, they will, after 
the Reflection, diverge from, or converge to another 
Point on the oppofite Side of that Surface, at the 
fame Diftance from it as the firfl ; and that a Line 
perpendicular to the Surface palling through one of 
thofe Points, will pafs through both. Hence it fol- 
lows, that if the Rays of Light emitted from any 
Point of an Objedt be fucceflively reflected from two 
fuch polifh’d Surfaces j that then a third Plane, per- 
pendicular to them both, palling through the emitting 
Point, will alfo pafs through each of its two fuccefs- 
live Images made by the Reflections: All three Points 
will be at equal Diltances from the common Interfedtion 
of the three Planes 5 and if two Lines be drawn thro* 
that common InterfeCtion, one from the original Point 
in the Objedl, the other from that Image of it which 
is made by the fecond Reflection : they will compre- 
U hend 
