Richard Towneley - Gascoigne's Micrometer

Gascoigne's Micrometer

Towneley published little of his own work but in May 1667 he sent a letter to the Royal Society touching the invention of dividing a foot into many thousand parts for mathematical purposes. Adrien Auzout had claimed a French first in inventing the micrometer. Towneley wrote to point out that Auzout was not the first person to have developed such a device. The English astronomer William Gascoigne had developed a micrometer before the Civil War. Towneley had produced an improved version of that micrometer and was using it in Lancashire. The Royal Society showed great interest in Towneley’s micrometer and he sent them one made in Lancashire by one of his tenants. Robert Hooke reported on it in November of the same year as A description of an instrument for dividing a foot into many thousand parts, and thereby measuring the diameter of planets to a great exactness with an illustration, reproduced here.

During the winter of 1664-5, the skies of the northern hemisphere were dominated by a brilliant comet, which was the most conspicuous since that of 1618. When Hooke made his first observations of the comet of 1664, he devised his own method of computing the angular diameter of the nucleus by comparing it with the apparent diameter of a weather vane support on distant building and measuring the distance between the telescope and the weather vane. Accurate angular measurements were of great importance to the astronomers of the time and Hooke realised he needed a precise instrument for this purpose. His problem was solved in 1667, when he saw Richard Towneley's micrometer, which was based on a prototype of 1640 invented by William Gascoigne. This instrument used a pair of fine-pitched screws to move two pointers in the focal plane of a Keplerian telescope. By enclosing the object to be measured between the pointers, its angular diameter could be computed to within a few arc seconds, providing the observer knew the exact focal length of the telescope and the pitch of the screw which moved the pointers. Hooke published an engraving of the instrument to accompany Towneley's description in 1667. Its principle was to lie at the heart of astronomical measurement down to the twentieth century.