In his book *Opticks*, published in 1704, Newton argued that light was made up of tiny particles called corpuscles. He based this idea on his observations of how light reflected and refracted, and how it could cast sharp shadows.

However, Newton's theory was challenged by Christiaan Huygens, who proposed a wave theory of light in 1678. Huygens' theory was eventually accepted as the more accurate description of light, but Newton's particle theory was not entirely forgotten.

In the early 20th century, Albert Einstein's work on the photoelectric effect revived the idea of light particles, which are now called photons. So while Newton was the first to propose a particle theory of light, it wasn't fully accepted until the 20th century.