LORD RAYLEIGH'S presidential address to the British Association, which is printed in full in a Supplement to this issue of NATURE, is divided into two distinct parts under two titles. The first part, dealing with “Vision in Nature and Vision aided by Science”, is a masterly general survey of developments in this field, which have taken place largely within the last two or three decades. In simple and direct language-itself a model of what such a survey should be-Lord Rayleigh describes the fundamental aspects of human vision and passes on to review the various ways in which the mechanism of the eye, employed as a means of observation, has been supplemented. The use of lenses, the telescope, the microscope, cathode rays, X-rays, spectroscopy and colour vision, photography and the sensitization of photographic plates, the photo-electric detection of radiation and television are all touched on, the salient features skilfully emphasized and the limitations and prospects of development duly noted. This part of Lord Rayleigh's address will surely bring home the vital fact that the largest part of our knowledge of Nature has been obtained by the sense of sight, and will remind us that progress by this means is not yet exhausted but bids fair, indeed, to go forward to fresh discoveries.