FOOTNOTES:
[1] Each memoir is separately paged and indexed.
[2] In order to examine Fig. [20], the book should be held with the left side of the page toward a window or lamp. The eye should also be at least two feet distant. The centre will then be seen to protrude, and the surface present the apparent section engraved below it.
[3] Messrs. De La Rue and Nasmyth, who used one of Mr. Lassell’s machines, as I have since learned, met with the same trouble, and were led to make two additions to the mechanism: 1, to control the rotation of the polisher rigorously; and 2, to give the whole speculum a lateral motion, by which the intersecting points of the curves described by the polisher were regularly changed in distance from the centre of the mirror. Mr. Lassell had previously, however, introduced a contrivance for this latter purpose himself.
[4] The glass that I have used has generally been such as was intended for dead-lights and sky-lights in ships.
[5] M. Foucault used plano-convex lenses of glass, of a radius of curvature slightly less than that of the mirror, and covered with paper on the convex face.
[6] By this it is not meant that there is a rippled polish, like that produced by buckskin.
[7] A right-angled prism cannot be used with advantage to replace the plane silvered mirrors, because it transmits less light than they reflect, is more liable to injure the image, and the glass is apt to be more or less colored. Its great size and cost, one three inches square on two faces being required for my purposes, has also to be considered.