The advance of photography : its history and modern applications (1911)

116 THE ADVANCE OF PHOTOGRAPHY properly annealed, so that the lens has no uniform action so far as light is concerned, or if striae are present, the defect is a much more serious one. These imperfections can only be found out by those whose eyes are practised in this kind of work, and even then it is necessary' to test with polarised light in order to be certain that the glass is not properly annealed. The striae may sometimes be seen, if present, by focussing the camera upon a bright light in an otherwise darkened room. The ground glass screen should then be removed, and the eye placed in the position of the image of the light : in this way the presence of striae can be detected. Stages in the Development of Lenses. — The perfection of the photographic lens has only been the outcome of a vast amount of practical and theoretical work by a large number of eminent scientific men. Not only had the material of which the lens is made to be considered, but a profound knowledge of mathematics was required in order to solve the problem as to the best shape of the lenses to obtain the desired result. In the early days of the nineteenth century (about 1830) Sir G. B. Airy worked out mathematically the theor} of the camera-obscura lens, and also turned his attention in a very thorough manner to the errors of astigmatism, and curvature of the field, to be met with in simple lens combinations. His work, however, seems to have been lost sight of for some time after the use of lenses for photographic work had become common. Later on, Petzval and Von Seidel independently solved the problem as to the nature of the material required for a lens (i.e. its physical properties of refraction and dispersion of light), and also the theoretical form of the perfect lens. In photographic work we endeavour to obtain on a flat surface a correct representation of a view in which the objects occupy many different planes, so that the lens