Lenses and Waves

1653 - TRACTATUS 43
exact description of the universe. 119 After all, the telescope was an artful
means to reveal new things in the heavens, whereas astronomical
measurement instruments aided the naked eye. 120 The step to combine these
two by aiding the artificial eyes with quadrants and the like, was not taken
immediately. Around the middle of the century, astronomical measurements
were still made by using the pre-telescopic methods and instruments
developed by Tycho Brahe. Hevelius used telescopes extensively to study the
surface of the Moon, but he turned to open-sight instruments when making
measurements. Efforts had been made to use the telescope for
measurements, but in vain. Until the 1670s, the accuracy of telescopic
observations was determined by the acuity of the human eye. But then
change set in. With the introduction of the micrometer, the telescope was
transformed into an instrument of precision. Significantly, the men closely
involved in that development were the ones to seek a more precise account
of the working of the telescope.
The configuration Kepler had thought up in 1611 had the drawback that
it reversed the image. Given the quality of lenses made at that time, it was
not advisable to add a third lens to re-erect the image. The two-lens
Keplerian telescope was therefore used only to project images, whereas the
Galilean type continued to be used for direct observation. In the course of
time the first advantage of Kepler’s configuration was discovered: its wider
field of view. When the length of a Galilean telescope is increased the field
of view quickly diminishes, which makes it very difficult to use. Towards the
1640s, the Keplerian telescope was gaining ground, in particular through the
good craftsmanship of telescope makers like Fontana in Naples and Wiesel
in Augsburg. 121 At some point in the early 1640s, the second advantage of
this type was discovered by the Lancashire astronomer William Gascoigne.
The Keplerian configuration has a positive focus inside the telescope; an
object inserted into it will cast a sharp shadow over the object seen through
the tube. Gascoigne relates that he discovered this by accident after a spider
had spun its web in his telescope. 122 Inserting some kind of ruler makes it
possible to make measurements of telescopic images. He died in 1644,
before he could publish his discovery and his measurements of the diameters
of planets. 123
Gascoigne’s accomplishments were made public in 1667 when Richard
Towneley, backed by Christopher Wren and Robert Hooke, claimed British
priority for the invention of the micrometer. This happened after a letter of
119 Van Helden, Measure, 118-119.
120 Compare Dear, Discipline and Experience, 210-216.
121 Van Helden, “Astronomical telescope”, 26-32. See also below section 3.1.1.
122 Rigaud, Correspondence, 46: “This is that admirable secret, which, as all other things, appeared when it
pleased the All Disposer, at whose direction a spider’s line drawn in an opened case could first give me by
its perfect apparition, when I was with two convexes trying experiments about the sun, the unexpected
knowledge.”
123 McKeon, “Les débuts I”, 258-266.