"Brilliancy and space-penetrating power, in the light of railway signals, is of the highest importance; and equally, or even more so, are the signals of steam-boats, etc.

'As the thickness at the centre of the lens of a railway signal lamp depends upon the distance of the lens from the flame of the lamp, in order that the di'vergent rays may be transmitted parallel; and as this distance is, on account of the limited size of the lamp, small, so the lens, as usually constructed, is very thick, generally from 2 to 2 1/2 inches in the centre; and being generally coloured red or some other colour the loss of light by absorption is very great.

Mallelt s Polyzonal Lamps 433Mallelt s Polyzonal Lamps 434

The figure represents a mode of obviating this, which I have adopted, by an application of the polyzonal lens of Fresnel. The lens is cast in two parts, - the outer or annular lens, and the inner or central one, then ground so as to have a common focus. The inner lens is fitted to the rebate of the annular one, by a little Canada balsam or other suitable cement; and thus the compound lens, at a small additional expense, has less than half the thickness at the centre of the common one, and as short a focal length with much greater penetrative power.

"Weight is also saved in the lens, which is of importance in railway signals, when mounted aloft in stormy weather, and it economises the coloured glass.

"The lenses, to be completely cleaned, must be washed outside to clear out dust from the sharp re-entering comers of the central lens, but even this might be avoided by fitting and cementing a circular disc of thin plate glass over the central space, from a to b in section, to keep out the dust."