CURIOSITIES OF LIGHT AND SIGHT. CURIOSITIES OF LIGHT AND SIGHT BY SHELFORD BIDWELL, M.A., LL.B., F.R.S. _WITH FIFTY ILLUSTRATIONS_ LONDON: SWAN SONNENSCHEIN & CO., LIMITED PATERNOSTER SQUARE 1899 PREFACE. The following chapters are based upon notes of several unconnected lectures addressed to audiences of very different classes in the theatres of the Royal Institution, the London Institution, the Leeds Philosophical and Literary Society, and Caius House, Battersea. In preparing the notes for publication the matter has been re-arranged with the object of presenting it, as far as might be, in methodical order; additions and omissions have been freely made, and numerous diagrams, illustrative of the apparatus and experiments described, have been provided. I do not know that any apology is needed for offering the collection as thus re-modelled to a larger public. Though the essays are, for the most part, of a popular and informal character, they touch upon a number of curious matters of which no readily accessible account has yet appeared, while, even in the most elementary parts, an attempt has been made to handle the subject with some degree of freshness. The interesting subjective phenomena which are associated with the sense of vision do not appear to have received in this country the attention they deserve. This little book may perhaps be of some slight service in suggesting to experimentalists, both professional and amateur, an attractive field of research which has hitherto been only partially explored. CONTENTS. PAGE. CHAPTER I. Light and the Eye 1 CHAPTER II. Colour and its Perception 39 CHAPTER III. Some Optical Defects of the Eye 84 CHAPTER IV. Some Optical Illusions 130 CHAPTER V. Curiosities of Vision 165 LIST OF DIAGRAMS. FIG. PAGE. 1. Image of Slit and Spectrum 12 2. Diagram of the Eye 24 3. Abney's Colour-patch Apparatus 45 4. Partially Intercepted Spectrum 49 5. Stencil Cards 52 6. Helmholtz's Curves of Colour Sensations 72 7. König's Curves 73 8. Stencil Card for Complementary Colours 77 9. Another form 79 10. Slide for Mixing any two Spectral Colours 80 11. Refraction of Monochromatic Light by Lens 87 12. Refraction of Dichromatic Light 89 13. Narrow Spectrum as seen from a Distance 97 14. Spectrum formed with V-shaped Slit 103 15. Bezold's Device for Demonstrating Non-achromatism of the Eye 108