Treatise on Light: In Which Are Explained the Causes of That Which Occurs in Reflexion, & in Refraction and Particularly in the Strange Refraction of Iceland Crystal

In 1690, one of history's most luminous minds turned his attention to a phenomenon that had baffled scientists for centuries: what IS light? Christiaan Huygens, the Dutch polymath who discovered Saturn's rings and invented the pendulum clock, here presents his revolutionary wave theory of light propagation, arguing that light moves through space as a series of waves in an invisible ether. The treatise systematically unpacks reflection and refraction through rigorous mathematics and experiment, but its most astonishing section tackles the strange case of Iceland crystal (calcite), which splits a single ray of light into two separate rays. This was not mere optics hobbyhorsing. Huygens was quietly challenging Newton's corpuscular theory, and his principle of secondary wavefronts would remain dormant until Young and Fresnel revived it a century later. Written in elegant prose that preserves the intellectual excitement of discovery, this is a front-row seat to the birth of modern optics from a man who refused to accept that nature's most fundamental behavior was beyond human understanding.