Prof. Snell on new articles of Philosophical Apparatus. 25 
and on the surface of milk or other white opake substance in a 
circular vessel. These, however, are inconvenient modes of ex- 
perimenting for the lecture room. A watch-spring, bent into a 
circular form and laid on white paper, serves a better purpose. But 
I have recently furnished myself with an instrument which pre- 
sents the phenomenon in its greatest beauty; and not only so, 
but by successive reflections produces caustics of several orders in 
the most perfect manner. I regard this little instrument as an 
important addition to our optical apparatus, and think it may not 
be unworthy of description in a public journal. 
I procured a steel ring, whose internal diameter was three and 
a half inches, made by bending and thoroughly welding a square 
half inch bar. Of course its external diameter was four and a 
half inches. A much less thickness between the inner and outer 
circumferences would, however, be sufficient. The interior was 
then turned in a lathe to as perfect a cylindric surface as possible, 
and highly polished. This is the essential part of the instrument ; 
but for convenient use it is mounted in the following manner. 
The steel ring (7,7), fig. 5, is enclosed in a ring of sheet-brass 
(aa), and by it secured to a disk of wood (6), from the back of 
which projects a brass stem; and this stem is united by a tight 
hinge-joint to the top of the brass pillar (c). ‘The whole stands 
firm on a heavy base of suitable size, as represented in the figure. 
The space within the ring is covered with smooth white paper, 
pasted down on the wood, so as to be perfectly plane. By turn- 
ing the base horizontally, and the hinge vertically, the face of the 
ring may be brought into any desired inclination with a sun-beam 
admitted into a dark room. At a certain inclination, the light 
reflected from one half of the cylindric mirror will be thrown 
down strongly upon the paper, forming the ordinary caustic 
curves, but far more delicate and true than I have ever seen them 
in any other mode of experimenting. These are marked (1, 1) in 
fig. 6. If the plane of the ring be less inclined to the beam, the 
rays will pass across to the opposite half, and after a second re- 
flection fall upon the paper in caustics of the second order, marked 
(2, 2). A still further diminution of inclination will reveal 
the third order (3,3); and in favorable circumstances I have 
seen the fourth (4, 4), very faintly and delicately traced. The 
general form of these figures is the same, but the size diminishes 
from the first order through all the higher ones; the position 
Vol. x11x, No. 1.—April-June, 1845. 4 
