468 Dr. C. Y. Drysdale on the 



to Mr. Cheshire *., in an article just published, Porro, in 

 1857 t, gave a very complete exposition of lenses and thin 

 lens combinations from the physical standpoint, including 

 suggestions for the study of spherical aberration and of thick 

 lenses. In Preston's i Light ' J a short account of the ele- 

 mentary treatment of lenses by this method appears ; and 

 Lord Payleigh § has also devoted attention to the subject. 

 It is, however, to Prof. S. P. Thompson || that we owe nearly 

 everything that has been done in recent years towards the 

 rational teaching of optics, and in 1889 he wrote a paper on 

 the curvature method of treating lens problems which he had 

 rediscovered and had used in teaching for about eight years ; 

 and he has followed this up by several valuable papers, and 

 his two important works — ' Optical Tables and Memoranda/ 

 and the translation of Lnmmer's ' Photographic Optics.' 



It will perhaps be well to mention here that the present 

 writer has unfortunately, until the last few days, been 

 absolutely unconscious of practically all that has been done 

 in the application of wave methods by others. Although a 

 student under Prof. Thompson in 1888-90, for some reason 

 none of this work was done, and the only thing that directed 

 his attention to the subject was a few lectures given by 

 Dr. Sumpner at the Central Technical College in 1892, 

 dealing with the matter as in Preston's 'Light.' The 

 interest roused by the manifest superiority of the wave 

 method, however, led him afterwards to make some simple 

 applications in practical work, and later to start a course of 

 lectures on technical optics at the Northampton Institute, 

 entirely based on physical optics. It is a singular fact that 

 although we owe to scientific men the wave theory of light 

 and its applications to optical theory, the optical trade have, 

 apparently quite independently, adopted a system of lens and 

 prism nomenclature which harmonizes completely with it, 

 and makes its application to reflexion and refraction problems 

 simple and logical. It was due to the writer becoming 

 acquainted with this notation that he was led to take up the 

 subject so fully, and he ventures to think that few teachers 

 of optical science would find a knowledge of optical trade 

 methods detrimental. 



! Before dealing witlr the curvature method, it may be inter- 

 esting to note that there are several alternative methods of 



* P. J. Cheshire, British Optical Journal, Nov. 1904. 

 t Porro, Societe Francaise de Fhotographie, vol. iii. pp. 211-222, 1857. 

 X Preston, ' Light,' pp. 99-106, Third edition, 1901. 

 § Lord Rayleigh, Encyclopedia Britannica, 1884. Article on "Optics." 

 I! S. P. Thompson, Phil. Mag. 1889, vol. xxxiii., "Notes on Geo- 

 metrical Optics," Part i. pp. 232-248. 



