1 The Birth of Science - MSRI

3
Other Hellenistic Scientific Theories
3.1 Optics, Scenography and Catoptrics
One of the first Hellenistic scientific theories was optics (), that is,
the “science of sight”, and the first known treatise on the subject is Euclid’s
Optics. In this work, Euclid deals with optics stricto sensu; according to
the nomenclature of the time (which we follow in this section), the term
included all that has to do with direct sight, but did not include reflection
(which was the object of another science, called catoptrics 1 ) or refraction.
The fundamental entities of the theory are visual rays (), which are
finite in number and extend in a straight line from the eye. The assumptions
of the theory establish simple correspondences between visual perceptions
and the beams of visual rays that intercept the objects seen. In
particular, they reduce the apparent size of an object to its angular dimension.
The emergence of optics illustrates well where the novelty of the scientific
method lies. The term “visual ray” had been in use for a long time 2
and the law that visual rays propagate in a straight line was well known. 3
Plato, recalling that the same object seems to be of different sizes depending
on how far it is from the viewer, states that no store should be placed page 83
1 Euclid’s Optics and a pseudo-Euclidean Catoptrics appear in [Euclid: OO], vol. VII.
2 Visual rays should not be confused, of course, with light rays. The relative roles of “visual
rays” and of light, both of which are necessary for seeing, is clarified in many sources, such as
Plato, Republic, VI, 507c–508a. Then as now, in order for an object to be visible two conditions must
be satisfied: the object must be lit and we must be looking at it.
3 Compare, for example, Plato, Parmenides, 137e, 3–4, where sight is implicitly used to define
straightness.
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