The non-linear projection of the retinal image has significant implications in both clinical ophthalmology and physiological optics, and was therefore considered by several (Prechtel and Wesley, I970; Mandell and St Helen, I97I) were considered in an attempt to formulate an adequate specification for a simplified schematic eye, and this was found for the most part to approximate to the earlier paraxial data of Cowan (I928) which was utilized in a wide-angle model by Stine (1934). It appeared necessary, however, to make some fundamental modifications. Accumulating data on corneal topography has supported the suggestion of Helmholtz (i 866) that the shape of the cornea is closely simulated by an ellipsoid. Such a surface can be specified in terms of an apical radius and an 'e' value, or eccentricity. The data of Prechtel and Wesley (1970), based on 150 eyes, show a clear unimodal tendency of this parameter which is further supported by the more detailed findings of Mandell and St Helen (I97I) on eight human eyes. The peak value from these papers was taken as e = o-, while retaining the apical radius of 7-8 mm. which results in the ellipsoid defined by the simple equation: x= l243 36-2y'It was also considered necessary to modify the data on the margin of the retina. This was located in accordance with the anatomical values quoted by Duke-Elder and Wybar (I96I ) on limbal distance of the ora serrata and the junction of the pars caeca with the functional retina. The resultant vertical and horizontal asymmetry of the retinal margin is shown in the Table, and is apparent also in the diagram of the optical geometry shown in Fig. i. The slope of the ellipse at any one point could be Address for reprints: N. Drasdo,

Aims-To devise a method to measure tear meniscus curvature by a non-invasive specular technique. Methods-A photographic system was devised. The system consisted of a camera and an illuminated target with a series of black and white stripes oriented parallel to the axis of the lower tear meniscus. The target was mounted on a flash gun close to the objective of a Brown macrocamera and calibrated using a graduated series of glass capillaries of known diameter, ground down to expose the inner wall. It was then applied to normal human eyes (n=45) to measure the tear meniscus curvature. A video system was also assessed which provided qualitative online information about the tear meniscus. Conclusions-Reflective meniscometry is a non-invasive technique providing quantitative information about tear meniscus shape and volume and of potential value in the study of ocular surface disease. (Br J Ophthalmol 1999;83:92-97) Results-Using

This is a review of studies that have investigated the proposed rehabilitative benefit of tinted lenses and filters for people with low vision. Currently, eye care practitioners have to rely on marketing literature and anecdotal reports from users when making recommendations for tinted lens or filter use in low vision. Our main aim was to locate a prescribing protocol that was scientifically based and could assist low vision specialists with tinted lens prescribing decisions. We also wanted to determine if previous work had found any tinted lens/task or tinted lens/ocular condition relationships, i.e. were certain tints or filters of use for specific tasks or for specific eye conditions. Another aim was to provide a review of previous research in order to stimulate new work using modern experimental designs. Past studies of tinted lenses and low vision have assessed effects on visual acuity (VA), grating acuity, contrast sensitivity (CS), visual field, adaptation time, glare, photophobia and TV viewing. Objective and subjective outcome measures have been used. However, very little objective evidence has been provided to support anecdotal reports of improvements in visual performance. Many studies are flawed in that they lack controls for investigator bias, and placebo, learning and fatigue effects. Therefore, the use of tinted lenses in low vision remains controversial and eye care practitioners will have to continue to rely on anecdotal evidence to assist them in their prescribing decisions. Suggestions for future research, avoiding some of these experimental shortcomings, are made.

No abstract

ABSTRACT.Purpose: To determine the effect of coloured light filter overlays on reading rates for people with age-related macular degeneration (AMD). Method: Using a prospective clinical trial design, we examined the null hypothesis that coloured light filter overlays do not improve reading rates in AMD when compared to a clear filter. Reading rates for 12 subjects with non-exudative AMD, associated with a relative scotoma and central fixation (mean age 81 years, SD 5.07 years) were determined using the Rate of Reading Test 1 (printed, nonsense, lower case sans serif, stationary text) with 10 different, coloured light filter overlays (Intuitive Overlays 1 ; figures in brackets are percentage transmission values); rose (78%), pink (78%), purple (67%), aqua (81%), blue (74%), lime-green (86%), mint-green (85%), yellow (93%), orange (83%) and grey (71%). A clear overlay (Roscolene # 00) (360 cdm-2) with 100% transmittance was used as a control. Results: ANOVA indicated that there was no statistically significant difference in reading rates with the coloured light filter overlays compared to the clear filter. Furthermore, chi-squared analysis indicated that the rose, purple and blue filters had a significantly poorer overall ranking in terms of reading rates compared to the other coloured and clear light filters. Conclusion: Coloured light filter overlays are unlikely to provide a clinically significant improvement in reading rates for people with non-exudative AMD associated with a relative scotoma and central fixation.