The Istituto e Museo di Storia della Scienza, Florence, Italy, possesses an astrolabe with five latitude plates that is now attributed to the Duisburg workshop of Gerard Mercator. Although it is known that Mercator made instruments, this is the first surviving example to be identified. Another latitude plate is shown to come from the workshop of the Florentine, Giovan Battista Giusti. A seventh plate, possibly engraved by Rumold Mercator, provides the only known Mercatorian polar stereographic projection. The role of (...) Egnazio Danti, cosmographer to the Grand Duke of Tuscany, in the acquisition of the astrolabe in about 1570 is considered. (shrink)

In a paper published in volume 50 of Annals of Science an astrolabe at the Istituto e Museo di Storia della Scienza, Florence, was attributed to the hand of Gerard Mercator, c. 1570, when his workshop was in Duisburg. This was the first scientific instrument by Mercator to be identified. Since then two further astrolabes by Mercator have been identified, one of them bearing his monogram: GMR. They belong to the Städtische Kunstsammlungen, Augsburg, and the Moravian Gallery, Brno. All (...) three instruments are described as a group, and reasons for believing that the Brno astrolabe was made earlier than 1550, and therefore in Louvain, are given. (shrink)

The concept of a technical frontier in branches of experimental measurement, such as the resolution of the microscope, angular measure and time telling, has been around for more than 60 years. The purpose of this brief paper is to identify the technical frontier operating on the achromatic astronomical telescope, where a limiting factor of the resolution of fine detail was the quality of the optical glass available. The achromatically corrected objective is formed from two kinds of glass, the common crown (...) glass and the heavy clear flint glass or lead glass. This last was difficult to make homogeneous, that is without regions of different density, and therefore different refraction and dispersion. Unusually, optical glass had to pass a second frontier, this time placed on the whole glass industry by the English Government in the form of excise duty, administered with a bureaucratic efficiency that effectively stopped, in around 1800, the making of optical glass suitable for the serious astronomical telescopes. The result of the tax imposition was to delay the English production of improved optical glass for more than 80 years. (shrink)

The general view is that there is one type of nocturnal, which is universal, first illustrated in a printed book in 1524. Recently, a number of quite differently constructed nocturnals has come to light. Six of these were made at the very beginning of the sixteenth century by Falcono of Bergamo in northern Italy. One of them, with the initials of the inventor, may well be the prototype. Five more are closely similar. Five further nocturnals of the same type have (...) been identified, but they are unsigned, and are not by Falcono. This paper establishes the existence of a hitherto unrecognized Italian workshop, and of a variant type of nocturnal. (shrink)

The present paper complements the publications of Gerard L'E. Turner on Mercator's astrolabes by presenting an account of an astrological disc which Mercator published at Louvain in May 1551. This instrument, of which only one copy is known, is described, and a transcription of its instruction sheet, with commentary and English translation, is provided. My preliminary study of the astrological content and context of the instrument indicates that it is connected with John Dee's astrological studies at Louvain from (...) 1548 to 1550. The didactic functions of the instrument are discussed, as well as the relationship between astrological theory and practice in Mercator's work. (shrink)