'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.
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.
"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."