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D.Phil.

M.Phil. / The anterior surface of the eye, especially the corneal curvature, is of importance in optometric and ophthalmic examinations. Pathological, surgical and contact lens induced changes to the cornea can have a significant influence on vision. It is therefore necessary to study the structure, curvature, and factors that influence the cornea. Each study contributes to a greater understanding about the nature of the anterior surface of the eye. Previous studies showed that physical exercise has an effect on the eye. These studies however looked at changes in intraocular pressure, visual acuity and other visual functions due to exercise. No study was found in the literature that investigated the effect of exercise on keratometric variation or on corneal curvature. A study done by Cronje-Dunn (1995) suggested the possibility of exercise influencing keratometric variation and the mean keratometric measurement. Due to little research in this area and the suggestions that exercise might influence keratometric variation, it was decided to investigate the influence of physical exercise on keratometric variation. In this study, 14 subjects took part in an experiment. The experiment consisted of three sets of keratometer readings obtained for each subject. The first set of readings was obtained before exercise. Directly after the first set of keratometer readings, the subject had to reach 85% of his predicted maximum heart rate on a stationary bicycle. Immediately after the cycling the second set of keratometric readings was obtained. After the second set of measurements the subject rested for an hour. The third set of keratometric readings was obtained after the hour of rest. The different sets of keratometric measurements were converted to h vectors. Transforming keratometric readings to h vectors and plotting the readings on threedimensional graphs representing symmetric dioptric power space make it possible to view the distribution or spread of the keratometric measurements and to define any variation in the measurements. The results from this dissertation indicate that keratometric variation and corneal curvature was influenced by physical exercise (cycling). Exercise increased variation in curvature around the vertical meridian and/or increased torsional variation in the horizontal and vertical meridians for most subjects. The increase in variation in curvature after exercise was greatest between the 80° and 120° meridian of the eye. This increase in keratometric variation after the cycling decreased after the rest period in most subjects. Little change in variation was noted in the curvital power around the horizontal meridian. Some subjects indicated a change in mean. The mean was greater around the vertical meridian either after the exercise and/or after the rest period.

D.Phil. / Under certain conditions the human visual system accommodates and postures at a position classically known as the dark focus. The dark focus is usually presented as a spherical phenomenon. The characteristics of the dark focus that can be conveyed by its spherical presentation are limiting: little representation can be given of the characteristics of the variation that occurs when measurements are taken of the dark focus (for example under dark conditions); no indication can be given of any antistigmatic (non-spherical) change or variation; differences in variation between light and dark conditions cannot be shown or detected using classical methods of analysis; meridional characteristics of the dark focus cannot be represented, and so on. In this thesis I have preferred to coin and use the term dark refraction shift (defined here as: 8F = F dark- Flight) for what has classically been known as the dark focus. I have done so because I believe that the term is a better description of what happens to the human visual system under dark conditions. Multivariate methods of analysis allow for a much more detailed, and complete, presentation ofthe dark refraction shift and its variation. The limitations of methods used previously and mentioned above are overcome when multivariate methods are used to analyze and present dark refraction shift data. This thesis presents the dark refraction shift and its various characteristics, making use of multivariate methods that are used for this purpose for the first time. A Hoya AR550 autorefractor, set to measure refractive state to the nearest 0.01 D, was used to take 50 measurements at a time of the refractive state of twenty subjects under varying conditions ofluminance. The autorefractor was re-focused after each measurement. In the light condition, the subject was positioned in front of the autorefractor, the room lights were left switched on and the fixation target inside the instrument was visible to the subject. 50 measurements were then taken under these conditions. A second set of measurements was then taken under the dark condition. In the dark condition measurements the subject was left in complete darkness for five minutes to allow the accommodative system to settle at the dark focus. No fixation target was visible to the subject who remained in complete darkness for the duration of the dark condition measurements. 50 autorefractor measurements were taken under these conditions. Two measurement sessions were conducted where the order of the light and dark conditions were reversed. In the case of each subject the order of light versus dark condition measurements was determined randomly for the initial session and reversed in the second session. Subjects had to fulfill certain selection criteria; the refractive state had to have a cylinder equivalent dioptric strength of less than or equal to 6 D, the visual acuity had to be 6/6 or better in the right eye, subjects with strabismus were eliminated and subjects with any observable ocular pathology were not accepted. Ten of the subjects were aged between 21 and 35 years of age. They constituted the prepresbyopic group. The other ten subjects were aged between 40 and 65 years of age and constituted the presbyopic group. Each subject had a total of 200 measurements taken, 100 measurements taken in the light condition and 1 00 taken in the dark condition.

M.Phil. (Optometry) / An investigation of the nature of variability or variation of refractive behaviour (in a sample of universi ty students studying optometry) is described. Measurements of refractive behaviour were obtained by means of autorefraction. This study was based upon multivariate methods of statistical analysis which have only recently become available in optometric science. Variation is examined using both quali tative and quanti tative methods including stereo-pair scatter plots, confidence and distribution ellipsoids, trajectories of change of dioptric power, meridional profiles, testing of hypotheses on means and variance-covariance, and graphs which represent the type of uniform variation in a 2-dimensional plane known as the i)-plane. These methods are of great assistance in developing an understanding of the nature of the variation shown, as well as, in developing an awareness of the distribution or spread of the population from which the sample was drawn. Analyses of variability of refractive behaviour on both an artificial, or test eye, and on several individual human eyes are also described. The significance of some important aspects of variabili ty of refractive behaviour involving normality and departures therefrom (such as results from outliers) are discussed and illustrated by means of examples. Distributions were found in which more than one mode was present (polymodal or multimodal behaviour). Distributions were also observed to vary from having an almost spherical spread of measurements (of refractive behaviour) to having a spindle or rod-like spread of measurements instead...

M.Phil. (Optometry) / Previous studies of corneal and keratometric variation used incomplete or incorrect statistical methods. For the first time, proper multivariate statistical methods are applied to evaluate short-term keratometric variation in human eyes. Keratometric variation is represented graphically by means of stereo-pair scatter plots, trajectories of change in dioptric power, ellipsoidal confidence regions for mean dioptric power, as well as meridional profiles. Quantitative expressions of variation are given in terms of mean values, variance-covariance matrices and volumes of 95% distribution ellipsoids. Manual and automatic keratometry is compared, both on a steel ball and on an eye. It appears that the automatic keratometer exhibits less variation than the manual keratometer....

M.Phil. / Using newly developed statistical analysis methods for refractive error this study examines the difference between autorefraction and subjective refraction and how it is related to age. The term autorefractive excess refers to the amount obtained by subtracting the subjective refraction from the autorefraction. The clinical sample consisted of five groups of fifty subjects each. The subjects in group 1 ranged in age from 1 to 10 years, group 2 from 11 to 20 years, group 3 from 21 to 30 years, group 4 from 31 to 40 years and group 5 from 41 years and older. Only one examiner (the author) and one autorefractor (Allergan-Humphrey 580) were used. The study found that there was a difference between the mean autorefractive excesses for the different age groups and that the difference between autorefraction and subjective refraction was statistically significant in both the left and the right eyes of all age groups. Left and right eyes exhibited similar behavior. The autorefractive excess for both the left and the right eyes together of group 1 was approximately -0.25/-0.18 x180 in conventional and (-0.25 0.00 -0.43)' in h-notation. The autorefractive excess increases by approximately (0.10 0.00 0.10)' per decade. There is an astigmatic component of approximately -0.18 x180 in both eyes of all age groups. Approximately 60% of the subjects had sphere-equivalent strengths of autorefractive excesses of under 0.50 D. Therefore 60% of subjects might be content with a prescription given from the autorefractor reading. Approximately 50% of subjects had cylinderequivalent strengths of autorefractive excesses of under 0.50 D.

M.Phil. / The aim of this study was to use auto refraction to investigate both the short-term intra-ocular variation of refractive status among primary school children and the spread of refractive status across eyes. The sample consisted of 90 Caucasian school children from a primary school in a small town, Eloff, in the Mpumalanga province of South Africa ...