The development of lens grinding and polishing techniques in the first half of the 17th century
Rolf Willach
When in 1608 the first telescope came onto the market in the Netherlands, the invention spread quickly all over Europe. The optical basis of the instrument consisted of simple spectacle lenses, manufactured using techniques known for over three hundred years. Only high quality spectacle lenses were suitable for telescopes, and only the very best of them could be used for making telescopes for astronomical application. The problem is well known, particularly from the correspondence of Galileo.
An investigation of telescope lenses of the first half of the 17th century clearly shows the serious lack of precision, and gives an interesting insight into the efforts of opticians to overcome these. The middle of the century brought an astonishing improvement in polishing techniques. Within only two decades the aperture of telescope objectives was increased dramatically. An invention by the Capucin monk Schyrl de Rheita was at the origin of this sensational progress.
An examination of telescope objectives of the middle and of the second half of the century shows that opticians from that time understood the main problems, and became increasingly able to cope with them. The second half of the century was the time of enormous progress in the use of new materials and techniques. We can identify most of the old object glasses by the modern techniques of optical analysis.
Micrometers for reading scale divisions
Dr Randall Brooks
In a previous paper (Journal for the History of Astronomy, 22 (1991) pp. 127-173) the historical development and use of the micrometers used in the optical path of telescopes from the 17th to 19th centuries were presented. This paper extends that history to include the very earliest micrometers and their application in the 17th and 18th centuries to reading the subdivisions of scales on the limbs of astronomical and surveying instruments such as quadrants, zenith sectors and theodolites. The instruments with scale micrometers devised by such individuals as Lucas Brunn, Christopher Treschler, Matthaus Heintz, Johannes Hevelius, Robert Hooke, Chevalier de Louville, John Bird and George Graham will be illustrated, including an assessment of the methods of calibration and use of the micrometers. The results of measurements made on selected scale-reading micrometers using modern equipment, with the consequences to our understanding of the accuracy of these devices and the astronomical problems that were being studied, will be presented.
A secondary standard in geodesy: the ‘Règles de Borda’
Dr Suzanne Débarbat
After the French Revolution (1789), the decision was taken – among others – to unify weights and measures. The new system was to be of decimal character, and from the new standard for length (as part of the circumference of the Earth), the standard for weights was to be issued. To insure a universal character, the ‘Mètre’ would be derived from a new measurement of the Earth between Dunkirk and Barcelona (1792-1798). Astronomical and geodetical measurements were needed. Latitude and azimuth determinations would be made using the Borda repeating circle used in 1787 for the linkage between the Paris and Greenwich meridians. For the measurements on the ground Borda devised new rules, the ‘Règles de Borda’ being at the same time bimetallic thermometers and length standards to be compared with the previous reference, the ‘Toise du Pérou’. After their use at the time of the law dated 10 December 1799, giving the definitive length of the ‘Mètre’, they were employed on several occasions during the 19th century. During the First World War, they were put in a safe place from where they came back in 1919. One can assume that they were also put in such a place on the occasion of the Second World War; but after the war, being forgotten, they were only recovered during the last few years. On the occasion of the bicentenary of the 1799 law, they were presented, at the Paris Observatory, to the public from September 1999 to May 2000. The history of these ‘Règles de Borda’ employed by Delambre and Méchain for the measurements of the two ‘bases’ included in their triangulation, giving them the status of a secondary standard, for the length of the ‘Mètre’, the ‘Mètre’ itself leading to the Metric System and to the Système International d’unités’ (SI), is recalled.
Some new sundials
Dr Allan Mills
The sundial has a long history, but nevertheless its development is far from exhausted. This claim will be illustrated by (amongst others) three recent examples: