Page:Harold Dennis Taylor - A System of Applied Optics.djvu/23

 Systems of Lenses, Perceval's Optics, etc., which are excellent as furnishing material for purely mathematical students working up for examinations ; but the manner in which the various problems are dealt with is in many cases ill adapted for application in practice, while certain matters of the highest importance are ignored altogether.

As a matter of fact there is not an English work on geometrical optics extant by whose guidance an ordinary photographic lens could be worked out in all particulars. Professor Silvanus Thompson's account of Von Seidel's system does not, however, give the impression that the latter's methods and notation are at all easy to comprehend, but certain it is that his system has been successfully employed for very many years by numerous mathematicians and opticians of the highest rank on the Continent, while the foundation-stone of English optical science has been left unbuilt upon.

I here allude to the all-important work which was done about thirty years before that of Von Seidel by Sir George Airy, and still more by Henry Coddington. Sir G. Airy published some highly important papers in 1827 in the Cambridge Philosophical Transactions on “The Spherical Aberration of Eye-pieces of Telescopes,” and another paper on the Achromatism of the same.

Then Henry Coddington took up the work, and by the aid of some very ingenious devices of his own contrivance greatly added to the simplicity and universality of the formulæ arrived at by Airy. In 1829 he published his labours under the title, A Treatise on the Reflection and Refraction of Light, which, although still the best work on geometrical optics from the practical optician's point of view, nevertheless contains many shortcomings, which I attribute chiefly to the fact that he had not had very much practical acquaintance with lenses and their properties. It is therefore with much diffidence that I venture to criticise and to supplement many of his methods and formulæ, especially when I feel sure that had it not been for his labours this treatise would never have been undertaken.

Another very important work on geometrical optics, now very little known, was Eichard Potter's Elementary Treatise on Optics, Part II. of which, published in 1851, contains certain formulæ for spherical aberration of the third approximation.

I may here state that the invention of the “Cooke” lenses for photography was not of a haphazard nature, but occurred in this way. I had been studying Coddington's work very carefully and did not feel quite satisfied with his method of working out the curvature