PREFACE. THE nomenclature of a science is one of the most im- portant portions of its structure, and the Science of Instruments requires a good dictionary as a basis. It is believed that a concise definition of each instrument, together with illustrations of the most important types, will be of real service to scientific men and the industry. This book is not only a means of information and reference respecting scientific instruments of every description, but also exhibits the great variety of such instruments that are produced in Great Britain. Key numbers corresponding with a numbered list of manufacturers are attached to the dictionary descriptions. The British Optical Instrument Manufacturers Association, under whose auspices the book is issued, is composed of the leading Optical Instrument Manufacturers of Great Britain, the names of whom will be familiar throughout the world. From these houses have emanated in years gone by the original types of which the presentday highly developed, and often complex, instruments are the outcome, and these houses have throughout maintained that high standard of excellence and fine working quality for which British instruments are famed. The British Optical Industry succeeded in meeting the increased requirements of the War to an extent that was not considered possible, and to-day with greatly enlarged factories, equipped with mechanical machinery of the most suitable and modern description, with supplies of optical glass constantly increasing in quality and variety, and aided by a Research Association adequately financed, it is in a position to give its supporters a service in brains and goods that has never before been available. The Association issues this book with the full assurance of the ability of its members to meet the demands of all who are interested in the promotion and maintenance of the supremacy ofthe British Optical Instruments, and invites their confidence and support. September 1919