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Clinical Optics and Refraction

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    Clinical Optics and Refraction. Ed by O J Lehmann, D H Verity, A G A Coombes, F G Ah-Fat, P J Francis, A C W Ionides. Pp 160, £19.99. Oxford: Butterworth-Heinemann, 1998. ISBN0-7506-2188-5.

    This book is aimed at the ophthalmologist, orthoptist, or optometrist, especially in their student or trainee days. It has an unaccustomed layout: each chapter is divided into (approximately) page sized chunks, with each page headed with a question asking which of the five statements about “topic X” is/are correct. There are then a few paragraphs summarising the topic, and at the bottom of the page are listed the answers as to whether the statements were true or false. The use of questions promises well as a learning tool, providing the well known pitfalls can be avoided: the statements must be well chosen to tap into the essentials of the subject rather than irrelevant trivia (measuring the critical angle of urine is a little obscure), ludicrous false statements must be avoided (“heterochromatic dimness matching” seems unlikely), the answers should actually be found by careful reading of the paragraphs, and (most importantly) all the answers must be correct. In general, the authors achieve this aim: there are a few notable exceptions, such as keratoconus being given as an example of index myopia. There are a few other occasions where the text itself contains mistakes: prismatic displacement is said to be greater for an object close to the lens. Any mathematical formulas relevant to the topic are presented (without derivation) in notes at the bottom of the page: these are obviously not considered particularly important—units of measurement are not always specified or are non-SI, the Cartesian sign convention attracts only cursory mention, and it was surprising to see u and v for object and image distance, rather than l and l′. None the less, the format works particularly well in a more applied topic: the discussion on aphakia can change quickly from aniseikonia to IOL calculations to contact lenses and their complications, all within a few pages. The book does not work so well as a primary source from which to begin to study a topic. Firstly, it does not have an index, and the chapter headings are broad and uninformative: it is not immediately obvious that “genetics of colour vision” will be in the chapter on refraction by the eye, or “blind registration” in clinical applications of optics; and “crowding phenomenon” is dealt under the heading “Ametropia” as well as “Strabismus”. The latter is a good example of where the lack of cross referencing is detrimental: another is the case of antireflection coatings mentioned in two separate “questions” in Chapter 1 and again in Chapter 2. This independent topic by topic treatment leads to repetition in the section on lasers: a summary table could have presented comparative data well. In other cases, tables are used to good effect, to summarise Purkinje image characteristics, for example. There is only one diagram in the book, and this is regrettable, particularly in sections relating to geometrical optics. Creating a word picture of the optics of retinoscopy is a challenging task, and the use of a logarithmic wavelength scale could elegantly convey the electromagnetic spectrum. The authors are to be congratulated on condensing a vast amount of information into a small space, and for dividing it conveniently into digestible chunks. It is a good book to read through before going into an exam, being a general refresher across a wide and varied syllabus. But to select specific topics and study them in depth, an alternative source will be needed.