William Gascoigne (1612-44) was an English mathematician and astronomer, renowned for making scientific instruments. He was intrigued by the vernier scale and saw its potential for measuring the angular distances between stars.
In circa 1635, while working on precision optics, he noticed that a thread from a spider's web had become trapped at the exact focal point of two lenses, and that he could therefore see it sharply. This inspired him to create a thin marker that could be placed at the focal point of a lens. He added a second linear marker so that, when he looked through his telescope lens, two parallel lines could be seen within the field of view.
One of the markers he linked to a very fine screw thread, which could be used to adjust the distance between the two markers—the other of which would remain fixed. As the angular size of the field of view of a telescope was known, Gascoigne's invention enabled precise measurements to be made of the positions of astronomical objects in the sky. Gascoigne's micrometer revolutionized accurate measurements in astronomy, but there were other uses to be discovered for this device, beside acting as a finely calibrated telescope sight.
British engineer James Watt, best known for his invention of the steam engine, adapted Gascoigne's idea in 1776 to produce a handheld micrometer screw gauge that measured actual sizes of small objects. By replacing the markers with callipers, and by knowing the size of the threads on the screw which adjusted them, he was able to add measuring wheels to the head of the screw. This allowed minute adjustments and measurements to be made. Watts's instrument significantly advanced the ability of manufacturers to make machine parts of precise dimensions.
