Miscellaneous Intelligence. 433 
It consists — of two conical mirrors of the same — and 
with a common axis, so placed that the beam of light i is first re 
ec; 
whence it is again reflected, in certain cases, parallel to its first direc- 
tion. Thus the first or ring-shaped beam of rays is condensed, so that 
it can be received on the object-glass of a telescope, in condition for ac- 
curate eee? the quantity of rays being greater as the ring-ra- 
dius is incre 
To apeesuate: this, we first observe that if a ray be ee, re- 
flected between two parallel mirrors, its directions, after all the even 
reflections, are parallel to each other. Let the two cones now es as- 
sumed, with a common axis and equal vertex angles, the inner surface 
of the outer cone, and the outer surface of the inner cone, being re- 
t 
First, take a beam of rays parallel to the common axis. Each ray 
of this beam which falls on the outer cone, and again, after reflection, 
on the inner cone, will emerge parallel to its primary direction, as the 
normals at its two points of reflection are parallel. Thus all the rays 
from the outer pp however great its radius be, will in this case be 
condensed into a new beam parallel to the first, but emerging from 
the inner cone. Mae object-glass, or rather a telescope, with its axis 
° 
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exceeds the area of telescope apert The condensing tones are 
thus — a perfect arrangement for increasing the intensity of light 
from a star on the axis. Cutting off from the inner cone just enough 
to cover the ‘abject: glass, and from the outer one the part supplying rays 
to that portion thus determined, and mounting the two asa sea 
appendage, light may be condensed to any extent, froma “star, by 
increasing the ring-radius. The effect would be idemical With that of 
enlarging the object-glass ot an area e equal to the orthographic ray pro- 
jection of the ring. This ring could doubtless be increased to twenty 
or thirty feet diameter, which with a two feet aperture would give 
such an illumination from an axis-star as is overpowering in contem- 
— 
, let a beam be taken, making, with the we a A 
nisl angle, ae as that of all beams in an astronomical field m 
always be. To trace this through the condenser, siopa the ring re- 
solved into elementary rings, and the beam into corresponding e ele ito 
i rmals 
mentary beam into the same plane. The rays fall parallel in revolu- 
tion, and their points of rebectioa fallin an are, subtending twice the 
angle between the axis of the beam and that of the cones. As the 
aberration due to the s re above revol 
have effected, the veils of second reflection will fall into a curve, dif- 
55 
Srconp Series, Vol. XVI, No. 48.—Nov., 
