26 NOTICES OF THE MEETINGS (Feb. 28, 
rays of the sun or moon; each wave may (in common with every 
curved surface) be considered as a polygon having an infinite number 
of sides: there must therefore be some side in such a position as 
will reflect the light. The same effect was produced by a row of 
glass rods placed side by side ; reference was also made to a looking- 
glass casting the sunshine on the wall,—to reflectors placed at a 
window to exhibit objects in the street,— to the glow in the sky pro- 
duced by a burning house; this appearance being half-way between 
the spectator and the conflagration, occasions continual mistake as to 
the locality of the fire. 
A. Argand lamps and reflectors.— Having exemplified the princi- 
ple upon which light is reflected, Mr. Cowper demonstrated, 
by means of a series of small mirrors which were moveable ona 
fixed axis, that if a light were placed in the focus of a paraboloid, 
the rays would be reflected parallel. This is done in those light- 
houses where reflectors are employed.—The difficulty of shaping 
paraboloids was referred to, and it was mentioned that they were 
raised from a flat sheet of metal by the hammer. The arrange- 
ment of the lamp and reflector was described ; and the halo-like dif- 
fusion of the light, consequent on the impossibility of concentrating 
the luminous point in the focus of the parabola, was noticed. 
In connexion with this part of his subject, Mr. Cowper, 
dwelt on the distinction between a fized and a revolving light. The 
former, being intended to be visible all round the horizon, requires 
more lamps than the latter ; when three rows of twelve lamps, each 
row being in contact, are arranged in a circular form, the three 
lamps which are in a vertical line immediately opposite to the spec- 
tator, afford a strong light, while the three on either side are less 
distinctly seen, the parallel rays described not reaching his eye. 
With respect to the revolving lights,— supposing 28 lamps ar- 
ranged on the four sides of a parallelopipedon ; then, as the figure 
revolves, each side will present seven lamps in succession. These, 
by shining at once, will produce a much stronger light than the fixed 
light. The duration of this effect will, however, be short ; because, 
as each side is turned away from the spectator, the light will decrease 
rapidly ; this will be succeeded by darkness, and this darkness will 
in its turn be dispersed by a rapidly increasing light. 
Mr. Cowper proceeded to state that so satisfactory had 
been the result of metal reflectors in lighthouses, that there 
seemed small scope for improvement, until Fresnel devised the 
application of lenses, and also reflecting prisms in combination with 
lenses, to a single large lamp. 
To make this invention clearly understood, Mr. Cowper ex- 
plained the general laws of the reflection of light, and illustrated 
his explanations by various diagrams and models. 
B. One Argand lamp, lenses, and reflecting mirrors.—Having shown 
that light on passing through a triangular prism of glass is refracted 
towards its base, Mr. Cowper applied this principle to the con- 
