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The object of this study is Leonhard Euler's physical optics as it is formulated in Nova theoria lucis et colorum (1746). The focus is on this particular work by Euler for two reasons: 1) Nova theoria represents undoubtedly the most comprehensive and systematic medium theory of the 18th century; 2) it contains the basic principles of Euler's conception of the nature of light, which he later maintained. The works of the most important advocates of this tradition (Huygens, Malebranche and Johann II Bernoulli) are here analyzed to give a historical frame to Euler's role in the medium tradition. Though these authors try to elaborate a theory of light alternative to the emission theory, they never realize the contrast between the medium and the emission traditions. From this perspective, Nova theoria is a real transition point: Euler is fully aware of the antithesis between the two traditions; he compares them, he refutes the arguments in favor of emission theory and formulates an alternative one, that will substantially be the first and the most significant antagonist of emission model. The essay examines also the central questions of Euler's theory of light, i.e. how pulses are generated and propagated, the nature of the rays of light and the relations among pulse distance, frequency, and velocity.