Newton's Experimentum Crucis Reconsidered

Abstract

Certain terminological inconsistencies in the teaching of optical theory at the elementary level are traced back to Newton's Opticks and shown to derive from an uncritical application of the terminology of the old, established Euclidean geometrical optics to experiments which are primarily concerned with the establishment of a physical theory of light. The “ray” of Euclidean geometrical optics should be considered as a geometrical operator, the working rules of which are laid down in a set of axioms. If the Euclidean ray concept is adapted to those dispersion experiments put forward by Newton in favor of his hypotheses, some general class properties of dispersion phenomena are revealed. From the insight in those class properties, certain counter experiments can be easily recognized which demonstrate that it makes no sense to connect a certain color of the ray with its degree of refrangibility. There is a brief discussion of the significance of such experiments for insight into the part played by pure observation in scientific theory making.

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