^{page 349 note *} Phil. Trans. 1838, p. 371.

^{page 350 note *} Opuscules Mathematiques, Tom. I. Mem. ix. p. 266.

^{page 351 note *} The two screens S, S′ may be the opposite edges fo a triangular notch in a card held in the hand.

^{page 353 note *} Opuscules Mathematiques, Tom. I. Mem. ix. § iv. p. 273–4.

^{page 354 note *} Phil. Trans. 1838, p. 388.

^{page 354 note †} June 1842, vol. li. p. 830.

^{page 359 note *} The fact of objects seen obliquely not being double, is ascribed by Mr Wheatstone to the coalescence of the images of different magnitudes given by each eye.

^{page 359 note †} Phil. Trans. 1838, p.386, § 14.

^{page 359 note ‡} Letters on Natural Magic, p. 54.

^{page 360 note *} See Letters on Natural Magic. Lett. III., p. 54.

^{page 361 note *} Several curious facts establishing this result have been given by Dr Smith in his Compleat System of Optics, vol. ii. 387–389; and Remarks, § 526–527.

^{page 365 note *} This account was published anonymously in the Edinburgh Journal of Science for January 1826, No. VII. vol iv. p. 97; and a popular abstract of it afterwards appeared in my Letters on Natural Magic, Letter V. p. 98.

^{page 365 note †} Phil. Trans. 1838, p. 383.

^{page 365 note ‡} This is true only when they are not seen obliquely.—D. B.

^{page 365 note §} Phil. Trans. 1838, p. 384.

^{page 366 noten *} When the cameo and intaglio are viewed very obliquely, one of the causes of deception disappears. In the case of a cameo appearing depressed, the depression disappears the instant that the shadow of the cameo encroaches distinctly upon the plane surface from which it is raised, because an intaglio never can, however obliquely viewed, throw a shadow upon the plane surface out of which it is excavated. For the same reason, an intaglio seen very obliquely will not rise into a cameo, because the shadow on the plane surface is wanting.

^{page 366 note †} Journal of the Rev. Joseph Wolff, 1839, p. 189.

^{page 366 note ‡} Vol. i. p. 334.

^{page 368 note *} Such as the magnified teeth of a saw, as in fig. 14, or a thin section of a hexagonal prism whose axis is parallel to a line joining the eyes.

^{page 368 note †} In order to produce simultaneously this double effect, the lines of the pyramid, for example, which are to give the converse of relief, should be fainter than the other lines, or in different and feebler colours.