In 1665 and 1666, Isaac Newton performed a series of experiments employing the prism, which radically transformed the traditional ideas on the nature of light and colors. He made a small hole in the window of his room, which had been completely darkened. A prism was arranged to intercept the beam of light entering the aperture and to project an image onto a wall several meters away. On the wall, Newton observed a spectrum that was not circular but elongated, displaying all the colors of the rainbow. Newton also showed the reversibility of the experiment: using a converging lens, the polychromatic spectrum could be made to generate the original beam of white light. From these experiments he deduced that colors were not accidental modifications of white light, as had been believed from Aristotle to Descartes. White light now appeared to be not an elementary substance, but a heterogeneous one, produced by the mixture of several colors. Newton correctly maintained that the spectrum appeared elongated because each colors had a different refraction index. This discovery led to the concept of the reflecting telescope, which could greatly reduce the visual fatigue caused by chromatic aberration. Newton published his findings in the Philosophical Transactions of the Royal Society in 1672 and 1675. The Newtonian theory of light and colors prompted lively debates. One of the most heated was the confrontation with the wave theory of light, advocated by Huygens.