( ' 4*5 ) 
- . -ft. * f •» T «-T Y 
IV. A Tropofition relating to the Combination of 
Tra.nl parent Lens’s with Reflecting Planes. 
By J. Hanley, Bj V. <Pr. S. Com- 
municated to the Royal Society, January 9, 
* 7 ? 4 - 
H AVING propofed the ufing of a Telefcope 
with the Inurnment for taking Angles, which 
I formerly laid before this Society, (.SVeN 0 420.) 
it gave me Occafion to confider the Effects of the 
combining feveral Kinds ofTelefcopes with reflect- 
ing Planes, and, among others, led me to the fol- 
lowing Proportion. 
That if two Lens’s of equal focal Length be 
put together in the Form of a Telefcope, and a 
Plane Speculum be placed before one of them, lb 
that the Axis of the Telefcope make any Angle 
with its Surface, and a Ray of Light (the Line of 
whole Direction lies in a Plane perpendicular to 
that Surface, and parting through the Axis of the 
Telefcope) fall on it, and be reflected from it, fo 
as to pafs through the Telefcope ; then the Line 
of its laft Direction, after parting the Telefcope, 
will make an Angle With that of its firft Direction, 
before its Incidence on the Speculum, very 
nearly equal to double the Angle made between 
the Axis of the Telefcope, and the Surface of the 
Speculum. 
V pH .O Z 01 i ; H3 0 0 01 ?J ' { A OJ 
LEM- 
