This foundational work on physical optics establishes that white light is not a simple, uniform entity, but rather a heterogeneous mixture of rays possessing distinct and immutable degrees of refrangibility and colorific dispositions. The author systematically dismantles earlier hypotheses—particularly the notion that colors arise from modifications of light caused by shadows or reflections—by employing ingenious prism experiments and cross-refractions to isolate, sort, and recombine simple, homogeneal light.
The investigation begins by defining the fundamental laws of reflection and refraction, proving that rays differing in color also differ proportionally in their refrangibility and reflexibility. Through a series of rigorous observations, the text shows that homogeneous light cannot have its color altered by subsequent refractions or reflections, confirming that the varied hues observed in nature stem from the differential separation and mixture of these primary ray components rather than any physical alteration of the light itself.
The second major movement of the treatise addresses the perplexing optical phenomena exhibited by thin transparent bodies, such as soap bubbles, water films, and compressed air plates between glass lenses. By measuring the precise intervals and thicknesses required to produce specific concentric rings of color, the author reveals a profound numerical analogy between the spacing of these thin films and the vibrational lengths of musical chords. This quantitative framework explains why uniform white light breaks into distinct chromatic fringes when viewed through a prism.
In the final sections, these principles are applied to natural bodies and large-scale atmospheric phenomena. The author presents a comprehensive mathematical explanation for the formation of primary and secondary rainbows through specific refractions and internal reflections within spherical raindrops. Furthermore, the work examines the structural causes behind the permanent colors of terrestrial objects and explores the inherent optical limitations of refracting telescopes.
Concluding that the chromatic aberrations of lenses cannot be corrected simply by altering their spherical contours due to the unavoidable dispersion of differently refrangible rays, the text proposes alternative designs. This theoretical progression ultimately leads to the development of reflecting telescopes, uniting physical theory, rigorous mathematical demonstration, and practical instrumentation into a unified science of light.
