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Proceedings of the liogal Society 
ray C, seems to promise to be of considerable use in observations of 
the sun’s atmosphere. A somewhat similar result maybe obtained 
by using a single large prism, one of whose faces, employed for 
total reflection, has a very slight cylindrical curvature. 
2. On a Method of illustrating to a large Audience the Composition 
of simple Harmonic Motions under various conditions. 
I have often felt the difficulty of illustrating, by means of Airy’s 
Wave Machine, and various other complex instruments of a similar 
character, the composition of plane polarised rays into a single 
elliptically or circularly polarised one; the difficulty arising chiefly 
in showing separately, but in close succession, to the audience the 
two vibrations which are to be compounded, and their resultant. 
Lissajoux’s apparatus would exactly answer the purpose if we had 
tuning-forks vibrating 10 or 15 times a second, its sole defect being 
the extreme rapidity with which differences of phase are run through; 
and, in fact, I have tried metronome pendulums with mirrors attached 
to them ; but I have since found the following arrangement to be 
much more satisfactory. It consists simply in using plane mirrors 
rotating about axes very nearly perpendicular to their surfaces. A 
ray reflected almost normally from each of two such mirrors, equally 
inclined to their axes, and rotating in opposite directions with 
equal angular velocities, lias communicated to it a simple harmonic 
vibration, whose line and phase can be adjusted at pleasure by a 
touch. Two such s}*stems of pairs of mirrors, connected by elastic 
bands with an axle driven by hand, enable the operator to illustrate 
every combination of two simple-harmonic motions, as well as of 
circular and elliptic vibrations. By an obvious adjustment it is 
easy to use, instead of equal periods of vibration, periods bearing 
any desired relation to one another; and by crossing one or more 
of the bands we reverse the direction of rotation in the correspond¬ 
ing shafts. It is absolutely necessary to have adjusting screws by 
which to regulate the inclination of each mirror to its axis. 
3. On a simple Mode of explaining the Optical Effects of Mirrors 
and Lenses. 
It is very singular to notice how small a matter makes the differ¬ 
ence between the intelligibility and unintelligibility of a demon- 
