FOOTNOTES:
[20] Field means the actual space covered by the lens.—E. S.
[21] Sometimes a singular regularity of curvature is noticed, and a spiral is formed closely resembling in configuration some of the great spiral nebulæ, as drawn by Lord Rosse, so that one is tempted to see in the centrifugal tendency of the disturbed water, and the centripetal effects caused by reflection from the basin’s surface, causes which may in some sense illustrate the laws operating in wider domains of space.