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From time to time in recent years we have read of the invention of new detectors of electromagnetic radiation. The performance of these is often compared to that of the human eye, e.g. a TV orthicon is stated to be ten times as sensitive as the eye. The figure usually refers to "detective quantum efficiency" (D.Q.E.) of the device, D.Q.E. being the ratio of information in the output of the device to that information contained in the image, and one is left with the impression that the human eye is about to become obsolete.However, the D.Q.E. of the eye is nearly constant over a range of luminance of 107, while the D.Q.E. of the best detectors peaks pretty sharply over a luminance range of only 102. The latter of course has very important applications, e.g. in astronomy, but designers of such detectors would very much like to achieve greatly increased working latitudes.Again, the human eye in spite of its inaccurate physical construction as an optical instrument records image quality in accordance with its theoretical potentialities. In one special type of observation it gives a resolution performance 15-20 times as fine as its normal expectation. To achieve these performances there are elegant eye-brain compensations and some of these are discussed: particularly interesting is the feed-back relating to sharpness of retinal images.Another aspect of compensation is of clinical importance to the vision practitioner. Ophthalmic tolerances should be assessed from the frequency distribution of refractive and oculo-motor anomalies of a fully representative population, which is not usually the case at the present time.In applied optics recently, image evaluation of optical systems has undergone a radical change: systems are now designed with integrated consideration of object field, medium and detector, with consequent improvements in results. The author enquires if a similar critical investigation of the time-honoured techniques of the vision practitioners could not be made. Certainly more attention is given these days to individual occupational visual requirements, but this is derived from the same input data. There is an awareness among some of the more thoughtful practitioners that the old techniques of prescribing could be improved, and consideration is given to one or two possibilities.