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The fact that the eye is an optical instrument is being constantly brought to our notice; we read it everywhere, we hear it discussed on all sides; and it is universally acknowledged. On the other hand, it cannot be denied that Physiology forms the basis of Medical Science, and that a thorough knowledge of the former on the part of the Practitioner constitutes an essential element of success in whatever department of Medicine he may be occupied.

We therefore venture to draw the conclusion that, in order to treat the eyes successfully, an intimate knowledge of their optical construction and their functions is indispensable.

Facts confirm our conclusions. At least two-thirds of the patients who consult an oculist suffer from optical disturbances. But even the Surgery of the Eye is closely connected with Physical and Physiological Optics. The most skilful operator will fail to realise the object of an operation for cataract, unless he is able to substitute a proper glass for the removed crystalline lens. A strabismus operation, when undertaken without an exact knowledge of the optical and muscular functions of the Eye, is but a rude and even dangerous experiment.

Manual dexterity alone no longer suffices in the treatment of Diseases of the Eye. In order to be a perfect operator, it is necessary besides to be able to correctly diagnose a case, and to crown one's work by the adaptation of the organ to its functions.

It will not do to commence the most unimportant article with the assurance that Ophthalmology has made enormous strides during the last three decades, and there to let the matter rest. In spite of all

progress, Practical Ophthalmology is still in its infancy. It belongs to us to follow our predecessors in a manner worthy of them, and to render productive their wonderful scientific achievements. This may be brought about not so much by the introduction of new ointments, collyria, and other remedies, as by a more intimate knowledge of the Physiology of the Eye, which deals in great measure with Optics and Physics.

It cannot be denied that the practice of the present day leaves much to be desired in this respect. This is no doubt due to the defective Physical and Mathematical education which is frequently received by the Practitioner, and explains why he instinctively avoids the study of anything bearing a resemblance to Algebra, and looks upon it as something very difficult to be mastered,—something quite without the confines of his sphere.

With regard to the study of the Refraction of the Eye, the amount of Mathematical knowledge which is essential in practice is, however, very easily acquired. Indeed, even in following the course of the rays of light through the entire dioptric system of the eye, as far as the retina-a calculation which, towards the end, becomes somewhat complicated—the Author has not found it necessary to go beyond the elementary knowledge of Mathematics which he acquired at college, when studying the obligatory Greek and Latin.

In order, however, to spare the reader even these formulæ, he has introduced a new plan into this book, and has separated the Mathematical portion from the rest of the work. The first chapter is specially devoted to those who take a particular interest in the scientific solution of the questions under consideration; its perusal, however, may be entirely omitted; the reader may begin with the second chapter, in and after which he will find no formulæ, and yet will easily understand the meaning of the text.

Even the Mathematical chapter is written in a simple and elementary way, not demanding of the reader any of that preliminary knowledge which is so quickly forgotten. The Author begins at the


beginning, with the luminous point emitting its undulations, which radiate in all directions. The latter are followed from one medium to the other through a plane, and then through spherical surfaces, which form systems more and more complex.

The work, however, is not intended to be a general treatise on Optics. In all that has been considered, the view of our special object_namely, the explanation of facts indispensable to the Ophthalmologist-has never been lost. Each theorem is followed by its practical application ; the single convex surface represents the cornea, the second medium the aqueous (or the vitreous) humor, the system formed by two spherical surfaces finds its analogue in the crystalline lens or in the spectacle glass; and the two systems combined form the dioptric apparatus of the Eye.

Thus it is to be hoped that the first chapter will enable the reader to dispense with other books on Physics, as far as concerns the resolution of the most essential problems in ocular Optics.

The indispensable Mathematical questions being thus treated, do not return to them. In this manner the other chapters of the work form an elementary manual on Refraction and Accommodation stripped of analyses and formulæ.

It has appeared to the Author that, in practice, Accommodation and Convergence, and the relations existing between these two functions, so necessary to binocular vision, have not sufficiently been taken into account. These, however, have become extremely simple to manage since the introduction of the metric system into Ophthalmology, which has given to the former as a standard the Dioptry, and to the latter the Metre-Angle. He has, therefore, not hesitated to develop these subjects to a greater extent than is usual in text-books.

A chapter on the Methods of Determination ought not to be wanting in this treatise on Refraction. But, as the Author is writing for practice rather than for science, he has only given those methods really applicable to the former. All the principles of Optometry are explained, but he has abstained from entering into a description of all the different kinds of Optometers. If the principle has been understood, it is easy to comprehend the manner of working and the practical value, not only of the Optometers of the present, but also of those of the future.

The clinical portion of the work comprises all that, from a practical point of view, the Author has found to be of importance in his own practice in the clinics of the different countries which he has had the advantage of visiting, and in the writings of competent authors. That there is still much to be added, to be perfected, and even altered, follows from the fact that the science of Ophthalmology is still in its infancy.

This work was first published in the French language. The present English edition has, however, been considerably modified by the Author's experience since its first publication.

The Author is greatly indebted to Dr. C. M. Culver, of Albany, U.S.A., who has spared no pains in the translation of this work, a service which he is happy thus publicly to acknowledge.


PARIS, May 1886.

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