With this feeling the author himself may look back without regret on his endeavours. From this consideration he can derive some encouragement for the prosecution of the remainder of his task; and although not satisfied with the result of his efforts, yet re-assured by the sincerity of his intentions, he ventures to recommend his past and future labours to the interest of his contemporaries and posterity. Multi pertransibunt et augebitur scientia. [1] Plate 1, fig. 3. [2] See Note C. [3] Some early Italian writers, Sicillo, Occolti, Rinaldi, and others, have treated this subject in connexion with the supposed signification of colours.--T. [4] The English technical expressions "flat" and "square" have an association of mannerism.--T [5] Towards the close of 1806, when Weimar was occupied by Napoleon after the battle of Jena.--T. NOTES. NOTE A.--Par. 18. Leonardo da Vinci observes that "a light object relieved on a dark ground appears magnified;" and again, "Objects seen at a distance appear out of proportion; this is because the light parts transmit their rays to the eye more powerfully than the dark. A woman's white head-dress once appeared to me much wider than her shoulders, owing to their being dressed in black."[1] "It is now generally admitted that the excitation produced by light is propagated on the retina a little beyond the outline of the image. Professor Plateau, of Ghent, has devoted a very interesting special memoir to the description and explanation of phenomena of this nature. See his 'Mémoire sur l'Irradiation,' published in the 11th vol. of the Transactions of the Royal Academy of Sciences at Brussels."[2]--S. F. NOTE B.--Par. 23. "The duration of ocular spectra produced by strongly exciting the retina, may be conveniently measured by minutes and seconds; but to ascertain the duration of more evanescent phenomena, recourse must be had to other means. The Chevalier d'Arcy (Mém. de l'Acad. des Sc. 1765,) endeavoured to ascertain the duration of the impression produced by a glowing coal in the following manner. He attached it to the circumference of a wheel, the velocity of which was gradually increased until the apparent trace of the object formed a complete circle, and then measured the duration of a revolution, which was obviously that of the impression. To ascertain the duration of a revolution it is sufficient merely to know the number of revolutions described in a given time. Recently more refined experiments of the same kind have been made by Professors Plateau and Wheatstone."--S. F. [1] "Trattato della Pittura, Roma, 1817," p. 143-223. This edition, published from a Vatican MS., contains many observations not included in former editions. [2] A few notes (marked with inverted commas and with the signature S. F.) have been kindly furnished by a scientific friend. NOTE C.--Par. 50.