The plates which generally accompany works like the present are thus a most inadequate substitute for all this; a physical phenomenon exhibiting its effects on all sides is not to be arrested in lines nor denoted by a section. No one ever dreams of explaining chemical experiments with figures; yet it is customary in physical researches nearly allied to these, because the object is thus found to be in some degree answered. In many cases, however, such diagrams represent mere notions; they are symbolical resources, hieroglyphic modes of communication, which by degrees assume the place of the phenomena and of Nature herself, and thus rather hinder than promote true knowledge. In the present instance we could not dispense with plates, but we have endeavoured so to construct them that they may be confidently referred to for the explanation of the didactic and polemical portions. Some of these may even be considered as forming part of the apparatus before mentioned. We now therefore refer the reader to the work itself; first, only repeating a request which many an author has already made in vain, and which the modern German reader, especially, so seldom grants:-- Si quid novisti rectius istis Candidus imperti; si non, his utere mecum. [1] The Polemical part. [2] This preface must have been written before the work was finished, for at the conclusion of the historical part there is only an apology for the non-appearance of the supplement here alluded to. [3] In the present translation the necessary plates accompany the text. CONTENTS Introduction xxxvii PART I. PHYSIOLOGICAL COLOURS. I. Effects of Light and Darkness on the Eye II. Effects of Black and White Objects on the Eye III. Grey Surfaces and Objects IV. Dazzling Colourless Objects V. Coloured Objects VI. Coloured Shadows VII. Faint Lights VIII. Subjective Halos Pathological Colours--Appendix PART II. PHYSICAL COLOURS.