John Marshall: the making of true spectacles.

In 1693 John Marshall of London devised a novel method of grinding batches of identical, good quality, lenses of a specified focal length. Its commendation by the Royal Society led to a trade war between Marshall and rivals in the Worshipful Company of Spectacle Makers. Despite initial opposition the method was rapidly adopted by London opticians and, though much modified, it forms the unrecognised basis of present day practice.

Spectacles improved to perfection and approved of by the Royal Society.

The letter sent by the Royal Society to the London optician, John Marshall, in 1694, commending his new method of grinding, has been reprinted, and referred to, in recent years. However, there has been no comprehensive analysis of the method itself, the letter and the circumstances in which it was written, nor the consequences for trade practices. The significance of the approval by the Royal Society of this innovation and the use of that approbation by John Marshall and other practitioners are examined. Gaps in existing accounts of Marshall's method are partly remedied by supplementing surviving written materials with accounts of contemporary, and present-day, trade practices based on his method. The reasons why Marshall and his contemporaries failed to record his method and specify his improvements are discussed. The reactions of the Spectacle Makers' Company and its more prominent members, both to the innovation itself and to the Royal Society's letter, are analysed. The impact of the new technique on contemporary and later opticians is described.

Thickness of the lateral surface of the temporal bone in children.

The placement of implantable auditory prostheses in children has raised questions concerning the thickness of the temporal bone in the region of implantation. The purpose of this study is to describe the thickness at specific sites of the lateral surface of the temporal bone in children of different ages. One hundred twenty-five intact temporal bones from 83 children of known sex, race, and age between birth and 20 years were measured. Thickness was measured with a specially designed micrometer at specific locations from a fixed reference point by use of a surface projected grid for site identification. Thickness was plotted against age on scattergraphs for each site, and regression analysis revealed a bimodal linear relationship. Sites medial to the temporalis muscle were the thinnest; sites associated with the posteroinferior insertion of the temporalis muscle, along the supramastoid crest, were the thickest. However, wide interindividual variability was the rule.

Growth of the lateral surface of the temporal bone in children.

The adaptation of implantable auditory prostheses for use in children has raised questions concerning both what effect growth may have on the implant and what effect the implant may have on growth. The purpose of this study is to describe temporal bone lateral surface dimensions in children of different ages to draw inferences about growth rates. Using a surface projected grid and point calipers, 158 intact temporal bones from 103 individuals of known ages between birth and 20 years were measured at six points from a fixed reference. These specimens were located in the skeletal collections of the Smithsonian Institute's Museum of Natural History, the Medical Museum of Walter Reed Hospital at the Armed Forces Institute of Pathology, and the Cleveland Museum of Natural History. Linear regression analysis helped clarify a bimodal pattern of growth in which marked increases in dimension occur from birth to 4 years of age and much less change is seen from age 4 to 20 years. These data suggest that implantation in children under 4 years of age is subject to maximal growth effects.

Safety margins for lead in the general population.

The results of monitoring blood lead concentrations, the accepted biological indicator for lead in man, are examined against the three trigger values put forward in the UK, first as justifying environmental investigation (25 µg 100 ml(-1)), second as justifying health checks (35 µg 100 ml(-1)) and third as likely to give rise to obvious symptoms in a few individuals (50 µg 100 ml(-1)). Arguments for using the proportion or the number of individuals above a trigger value rather than the ratio of the mean blood lead concentration to the trigger value, the conventional safety margin, are presented. The numbers of individuals or proportions in the total population who are likely to be above the trigger values have been estimated and shown to be relatively small for all three. Factors likely to affect blood lead concentrations are examined against the possible effects of the changes, current and proposed, in the controls imposed on lead usage and on pathways. The qualitative changes expected are considered by groups-eg. smokers and drinkers, by regions, eg., those with lead-free drinking water as against those still with lead in their supplies, and for the general population eg. from the elimination of the soldered can for food and the reduction, and eventual elimination, of lead in petrol.Because the relationship between intake and blood concentration is non-linear in the UK, those with the highest existing blood lead concentrations in the general population must be expected to show the smallest relative reductions in blood lead for any small reduction in exposure. The analysis also concludes that those at highest risk will have to depend on their being identified individually and action taken on them and their immediate specific environment.

Towards a scientific basis for regulating lead contamination.

Dangers from exposure to substances released to the environment, subsequently discovered to be hazardous, have been limited as a rule by actions taken piecemeal. Recently, more analytical techniques have been devised under the title 'Risk Analysis' or some similar name in order to produce a more scientific basis for establishing environmental regulations, but the results are often too complex to use. Their application to one substance, lead, is made practicable, although still leaving a difficult task, by making two assumptions: first, that blood lead concentration in humans may be taken to be an indicator both of total exposure and of its hazards to human health, and second, that 'threshold' values of blood lead concentration above which some action is necessary can be set and adopted. Programmes to control lead usages and pathways in order to reduce exposure in value-effective, acceptable and enforceable ways can then be devised and implemented, together with monitoring programmes to check the results and effectiveness of controls. Adopting these two assumptions offers the advantage that should later research show that threshold values have been set too low or too high, the rate of applications of all or any of the individual control programmes can be modified accordingly. The method is illustrated by using the distribution curve of blood lead concentrations now available for the general population of Great Britain. This is first used to estimate the numbers above each of the agreed threshold values: this procedure establishes the size or the importance of the problem. The effects on numbers and the sizes and speeds of the responses, of each of the different ways of controlling the different sources and pathways, are then assessed qualitatively, and their feasibility, enforceability and costs reviewed. The monitoring programme to check the effectiveness of the controls so far imposed is described. By using this procedure, lead contamination can be regulated to protect the health of the general population and the results of any or all of the regulations reviewed. One immediate consequence from its general application is to demonstrate that separate and specific measures are necessary to protect those at highest risk. One immediate advantage is that the procedure identifies those decisions that require value judgements.

Integrated monitoring - Principles and practice.

The concept of integrated monitoring is examined and defined. The different types and applications of monitoring in the United Kingdom are considered against the information they can provide. Two examples of integrated monitoring of health hazards in the UK are described. The relevance of the principles and the practice to the assessment of regional and global environmental problems is discussed and suggestions made for their review.