OF ILLUMINATING LIGHTHOUSES. 
225 
P.S. Since writing the above, it has occurred to me that a considerable increase of 
illuminating power would be obtained in a reflector whose focus was in the plane of 
its end by the application of a spherical reflector (as in the annexed figure) to in- 
tercept the rays which would otherwise be lost, and thus to return them through the 
light itself, and thereby increase its intensity. 
The spherical reflector ought to be made about one fifth 
of the diameter of the parabolic reflector, and a little less 
than a semisphere, so as just to be without the lines A C. 
Here it will be seen that the light upon the part d d will 
be doubled through all its divergences, minus the absorp- 
tion of light in the spherical reflector, and the area of light 
obstructed by it. If the spherical reflector were made of 
glass silvered (in which I find from experiment, that the 
absorption of light is about two fifths,) the increase of illu- 
minating power obtained by this means in the parabolic re- 
flector would be about one third or one fourth. 
The effect of a coloured light might also probably be pro- 
duced by a similar segment of coloured glass between the 
lamp and reflector. 
