CORNEAL CURVATURE EROM PROFILE MEASUREMENTS*

The determination of corneal curvature from profile photographs has received scant attention in the past history of research into corneal topography. This paper describes the principal elements of experimental procedures involved in profile measurements and summarizes some experimental work being undertaken to eliminate the validity and potential of the technique.     

A Methodological Study of a Nonlinear Stochastic Model of an AIDS Epidemic with Recruitment

A nonlinear stochastic model of an AIDS epidemic with recruitmentof infectives, susceptibles, and AIDS cases into a randomlymixing population of male homosexuals was formulated and studiedfrom a methodological point of view through intensive computerexperimentation. Probability generating functions were usedto formulate a model for the monthly probability that a susceptibleindividual becomes infected with HIV, under the assumption thatthe probability of infection per sexual contact varies as afunction of the duration of infection. A method for taking intoaccount the use of condoms to prevent infection with HIV wasalso introduced. Nonlinear difference equations, resemblingdeterministic epidemic models, were embedded in the stochasticpopulation process by iterating an initial conditional expectation.Examples of Monte Carlo experiments are presented, illustratingthat solutions of these nonlinear difference equations are notalways good measures of central tendency for variations in thesample functions of the process. Two important substantive conclusionsdrawn from the Monte Carlo experiments were that efforts shouldbe made to collect quantitative information on the probabilityof infection per sexual contact as a function of duration ofinfection and the frequency of condom use within and among riskcategories in a population.     

RESPONSE OF ARTERIAL SEGMENTS FROM GRAVID HUMAN UTERUS TO MULTIPLE CONCENTRATIONS OF LIGNOCAINE

Nine uterine artery segments were obtained during Caesareanhysterotomy. The amount of isometric tension developed by eachsegment when exposed to 3, 6, 10, 20, 50, 100, 1000 and 2000-µg/mlof lignocaine hydrochloride (Xylocaine) was measured. All ninesegments contracted in response to the 1000- and 2000-µg/mlconcentrations. Eight of nine, six of nine, and three of nineresponded to the 100-, 50- and 20-µg/ml concentrations,respectively; one segment contracted in response to all concentrations.A dose-response curve indicated that maximum responses wereobtained at the 1000-µg/ml concentration (0.1%). Althoughlocal anaesthetics are usually considered vasodilators, thisstudy indicates that lignocaine causes the uterine artery toconstrict in concentrations in excess of 20µg/ml. Suchconcentrations might occur after paracervical block or accidentalintravascular injection, but would not be expected in associationwith extradural analgesia.     

PREDICTING THE ROTATIONAL PERFORMANCE OF TORIC SOFT LENSES

The major problems of meridional location in soft toric lens prescribing are examined The use of standard soft lens trial sets in the prescribing of toric lenses has been investigated and found to be of no value in regard to predicting the lens rotation of a prescribed toric lens. Appendices are included dealing with a theoretical interpretation of lens rotation as well as a consideration of methods for the determination of meridional location of soft lenses.