W. S. STILES AND THE VARIATION IN DIRECTIONAL SENSITIVITY OF THE RETINA

The death at the age of 84 of Walter Stanley Stiles, PhD, OBE, FRS, on 15 December 1985, robbed Britain of one of its most distinguished visual scientists. Stiles speht almost all of his professional life working in the Light Division of the National Physical Laboratory at Teddington, Middlesex. Although perhaps most familiar as the co-discoverer of what is now known worldwide as the Stiles Crawford effect, his contributions to psychophysics, made during an active scientific life which spanned more than half a century, covered many other fields. Much of his earlier work was concerned with the parameters that affect glare and, in conjunction with B. H. Crawford, he introduced the concept of veiling glare. This interest continued through a long series of papers on street and vehicle lighting, published during the thirties. As Weale (1986) notes, he started to grapple with the nature of the quantum stimulus (e.g. Stiles, 1932) before the work of Hecht, Shlaer and Pirenne (1942). He also made an early attempt to determine a density difference spectrum for the living human retina using fundus reflectometry, although here he was hampered by his reliance on visual photometry rather than on the photomultipliers and other photoelectronic detectors that greatly simplify such studies today. Undoubtedly some of his major contributions were to the study of colour, particularly through the development of two-colour increment threshold methods and his associated nmechanisms, and wide-field colour mixture functions. Indeed, he continued to be active in this field into his eighties, working with Gunter Wyszecki (alas, also recently dead) at the National Research Council at Ottawa, Canada. Most of his widely scattered original articles on colour are, fortunately, collected in the book Mechanisms of Colour Vision, which also includes his own retrospect and comments on recent developments in this area (Stiles, 1978). His wider views on colour are incorporated in Colour Science, which he co-authored with Wyszecki (Wyszecki and Stiles, 1967). Illustrative of his interest in optometric topics were his papers on the photometry of ophthalmic test objects (Stiles, 1944) and his discussion of spectral sensitivity curves at the 1948 Optical Convention of the Worshipful Company of Spectacle Makers (Stiles, 1948). Nevertheless, it is probably the Stiles Crawford affect, with its implications for the retinal architecture and its optics, that is probably of most direct interest to those concerned with the establishment and maintenance of good standards of visual acuity. The story of the discovery of the Stiles-Crawford effect is an interesting one (Stiles and Crawford, 1933). It arose from an attempt to develop an apparatus for measuring the area of the eye pupil, based on what was then the widely held assumption that the apparent brightness of a uniform field would be proportional to the pupil area. The investigators were surprised to find that their apparatus failed to yield estimated pupil diameters which were greater than 5.5 mm, even though they knew that the pupil had dilated to 8 mm diameter. Unlike many of us, who might well have been disposed simply to abandon an apparently inadequate piece of apparatus, Stiles and Crawford carefully pursued the source of this discrepancy and were able to show that i t arose from the fact that light which entered the centre, as compared to the periphery, of the eye pupil provided a more effective visual stimulus. They then went on to explore in meticulous fashion the effects of wavelength, retinal location and the level of adaptation. While, not unnaturally, the effect was initially seen as being primarily of importance in the applied context of photometry, a topic of obvious relevance to the work of the Light Division at the National Physical Laboratory, it soon became clear that its origins were likely to be of fundamental significance in attempts to understand the mechanisms of vision. In their first paper on the subject, Stiles and Crawford (1933) were able to show that more obvious explanations in terms of absorption and reflection in the optical media of the eye were unlikely to be adequate and that retinal factors were probably at least partly responsible. [ In fact Weale (1961), Mellerio (1971) and Vos and Van 0 s (1975) have since shown that the longer axial path through the blue-absorbing pigment of the crystalline lens tends to reduce the Stiles Crawford effect at shorter wavelengths. ] An indication of the possible form that a retinal explanation might take was given by Wright and Nelson ( 1 9 3 6 ~ who concluded their paper of measurements on the effect with the speculation that “The light may be totally internally reflected inside the walls of the cone and when the angle of incidence becomes too great the light escapes outside the cell and is inactive”. Interestingly (and perhaps inevitably) the great Helmholtz had earlier discussed this possibility in his monumental Physio/ogica/ Optics, although Wright and Nelson d o not seem to have been aware of this. Helmholtz (1962) wrote: “Briicke called attention to the peculiar advantage which must be afforded by the bacillary layer of rods and cones in the reflection of light from the retina. These elements are small cylinders, 0.030 mm long and 0.0018 mm thick, made of some highly refracting substance. Packed closely together like palisades, they constitute the last layer of the retina. The axes of those which cover the retina in the back of the eye are pointed towards the pupil, so that all light that falls on these elements penetrates them nearly parallel to their axes. Now when light, proceeding in a denser medium, arrives at the boundary of a less dense medium at a very large angle of incidence, i t is totally reflected there. So we may infer