D. A. Barnes

Posterior corneal surface toricity and total corneal astigmatism

The influence of posterior corneal surface toricity upon total corneal astigmatism was investigated in 60 young subjects (mean age 22.04 ± 3.24 years). Both surfaces were found to be flatter horizontally than vertically. Astigmatism arising from the anterior corneal surface was therefore reduced by astigmatism of the opposite sign arising from the posterior surface. Had the toricity of the posterior corneal surface been purely governed by that of the anterior surface this reduction would have amounted to about 5%. However, the posterior surface was found to exhibit additional toricity bringing about a greater reduction of total corneal astigmatism amounting to approximately 14%.

Normal variations of the posterior corneal surface.

Abstract. Measurements of the sphero‐cylindrical components of the rear corneal surface were taken from 80 healthy right eyes in order to determine normal variations. Dimensions of this surface were strongly influenced by the front corneal surface with the exception that the rear surface exhibited more toricity. Both surfaces tended to be flatter in males compared to females and in myopes compared to hyperopes. The corneal surfaces were also found to be flatter in younger eyes compared to older eyes, but this finding was most likely due to the preponderance of myopes in the young and hyperopes in the older group. The influence of the ratio of anterior: posterior corneal surface radius upon the estimation of total corneal power as required for intraocular lens inplant calculations was also considered.

Peripheral astigmatic asymmetry and angle alpha

The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R‐1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images 1 (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 ± 7.0° (Hartinger) and 9.4 ± 9.8° (Canon). Both values were found to he significantly higher than angle alpha 5.0 ± 1.2°. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.

A REVIEW AND EVALUATION OF THEORIES OF REFRACTIVE ERROR DEVELOPMENT

Abstract A review of theories of the development of refractive errors is presented. These are classified into three major groups: the biological – statistical theories, the use – abuse theories and theories of emmetropization. They are commented upon and their merits contrasted. Current research developments which relate to these theories are discussed.

Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques

A method for measuring the radius of the posterior corneal surface using the first and second Purkinje images has recently been developed. Values for the posterior corneal radius in the vertical meridian obtained using this method are compared with those obtained using a more conventional slit lamp method. A good correlation was found between values obtained using the two methods.

Measurement of posterior corneal surface toricity.

We describe a method in which posterior corneal surface toricity can be determined by photographing catoptric (Purkinje) images in three fixed meridians simultaneously using infrared light sources. Keratometry, the comparison of anterior and posterior corneal catoptric (Purkinje) images and pachometric results, are applied to three meridional analysis equations, allowing the posterior corneal surface to be described in spherocylindrical form. The technique is evaluated on five subjects with anterior corneal radii varying from 7.11 to 8.52 mm.

A study of the axis of orientation of residual astigmatism

Abstract. The aim of this study was to determine the axis of orientation of residual astigmatism in a sample of human eyes applying the principle of astigmatic decomposition. Calculations were carried out on keratoscopic and refractive data collected from the right and left eyes of 70 subjects (37 male and 33 female students) of mixed race (including 25 Asians and 43 Caucasians). No statistically significant difference was found for mean levels of residual astigmatism measured in the right (0.46 DC × 98.2°) and left (0.50 DC × 99.4°) eyes. Residual astigmatism was predominantly against‐the‐rule (83% of right eyes and 66% of left eyes) and was within ± 20° of being perpendicularly disposed relative to the corneal astigmatic power axis in two thirds of the eyes measured. No statistically significant differences were found for either gender or race.

An evaluation of Bennett's method for determining the equivalent powers of the eye and its crystalline lens without resort to phakometry

Refraction and optical components have been measured in 36 eyes using objective optometry, keratometry, ophthalmophakometry and A‐scan ultrasonography. A computing scheme was used to confirm that the accumulated experimental errors for a phakometric technique tend to exceed the inherent errors of a new method which calculates the equivalent powers of the eye and its crystalline lens without resort to phakometry. Bennetts method was shown to be a valuable technique in biometric studies which obviates the need for lengthy experimental investigations.

A METHOD FOR RAYTRACING THROUGH SCHEMATIC EYES WITH OFF-AXIS COMPONENTS

Abstract A linear algebraic method for tracing skew rays through aspheric surfaces is described. The advantage of this method is that the refracting surfaces can be translated and rotated with respect to each other. The method is used to investigate the optical factors affecting the location of Purkinje images.

Modelling oblique astigmatism in eyes with known peripheral refraction and optical dimensions

The optical dimensions of 34 young adult eyes were measured by optical and A‐scan ultrasound methods. Peripheral refraction was recorded up to field angles of 40 degrees to produce results for oblique astigmatism. These findings were modelled in schematic eyes with a monoindicial crystalline lens but with varying levels of asphericity of the optimal surfaces.