2000 HSS/PSA Program 1 - History of Science Society

Yaakov Zik University of Haifa, Israel
HSS Abstracts
Beyond the Naked Eye
Historians of science commonly describe Kepler as the father of modern
geometrical optics. Kepler clarified the foundation of optics and solved a series
of problems that had confounded his predecessors: the problem of vision, the
camera obscura and the geometrical theory of the telescope. Kepler’s books
on optics, Ad Vitellionem Paralipomena (1604), which is solidly based upon a
medieval foundation and Dioptrice (1611) which is considered to be the first
treatise in modern geometrical optics, are the mile stones of his optical work.
However, careful analysis of related texts and events, will clearly show that
something in the above description is missing. In 1604, while Kepler was
working on the Paralipomena, he disparaged the telescope most vigorously.
The first time Kepler understood the potential of this instrument as a scientific
device was after he had read about Galileo’s discoveries, published in the
Sidereus Nuncius (March 1610). Kepler wrote the Conversation with Galileo’s
Sidereal Messenger (an answer to the Sidereus Nuncius) in April 1610 and
from the way he described the construction of the telescope, it was clear that
he had not understood the importance of the focal length ratio and how the
magnification was determined. By the end of August 1610 he got a telescope,
made by Galileo, with which he was able to see Jupiter and its satellites. In
late September 1610 Kepler completed his Dioptrice (which was published in
the beginning of 1611). The chronological course of events strongly suggests
that Kepler’s reading of Sidereus Nuncius was not enough for him to understand
the optical principles of the telescope. Only after examining and employing a
telescope made by Galileo, was Kepler able to exploit the technical details
and to learn how it works. It appears that the whole story deserves a reexamination.
On the one hand it seems that Galileo must have known something
about the science of optics and that we have underestimated his knowledge at
our peril. On the other hand Kepler needed the knowledge that Galileo had
attained, as reflected by his inability to construct the telescope, much more
than what historians of sciences have been willing to accept. In this paper I
shall argue that there are more key figures in this story and that its various
aspects have not been properly appreciated up to this very day. Moreover,
most of the theoretical aspects regarding the phenomena of light, vision, and
lens properties necessary for the construction of the telescope (with the
exception of the law of refraction which is in any event not essential to our
case), had been at hand even before the time when Kepler addressed himself
seriously to optics. I shall claim that over and above the actual building of the
telescope it was Galileo who put all the theoretical pieces together. In my
paper, I shall present a few aspects of the claim that in Galileo one finds a
fusion of practical and theoretical knowledge which is crucial for the
construction of an instrument that is also a bearer of theory.
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