November 10 1619, was a life-changing night for then 23 years old French polymath Rene Descartes. He attended the coronation of Ferdinand II, the Holy Roman Emperor in September and was returning to join the Catholic army of Maximillian in Bavaria. Maximillian was preparing for a long war that would last for decades. Due to a harsh winter storm, Descartes had to stop in the small town of Ulm near Munich. That night, he sees three distinct dreams. In the first dream, he escapes from the horrors of phantoms who followed him. In the second one, he wakes up by a loud noise and sees sparkles of light in the room. In the third one, he sees two books appearing on the table: an encyclopedia that is incomplete and a book of poetry. He is rejoiced by immediately realizing that the books are pointing him towards his life purpose. He concludes that the dreams have set him to reform all human knowledge. 

There is an analogy between Descartes dreams and the allegory of the cave. Descartes escaping from the phantoms symbolizes the horrible turmoil and folly of the time, which is the state of soldiers who are sent to fight for the religion or the slaves in the cave. The second dream points to recognizing the false sources of light or the fire in the cave, which are the confusing and doubtful beliefs of the time. The third dream gives him hope and shows him the way to grasping the Primordial Light of knowledge.

Ten years later, Descartes lives in the Netherlands hiding himself from the seemingly unending wars. He dedicates all his time to his theory of metaphysics. A rare phenomenon happens in Rome. People see multiple suns in the sky. The phenomenon is called Parhelia or sun dogs. Descarte is fascinated by this event. He abandons his treatise and instead works on another project called “The World, or Treatise on Light”. Treatise on light is the most visionary work of Descarte and one of his most influential works.

That is how the inquiry about the nature of light started again with much caution. It was yet very difficult to challenge any established belief and very easy to step on the wrong side of the argument. Rene Descarte in his Treatise on Light begins by: “I know of only two sorts of bodies in the world in which light is found, to wit, the stars and flame, or fire. And, because the stars are without a doubt farther from human knowledge than is fire or flame, I shall try first to explicate what I observe regarding flame.” Let’s first talk about things that we can personally examine. Descartes believed that what is true must be clearly conceived. It might not be much appreciated these days, but it was a revolutionary attitude at the time. He goes on to explain his idea of the mechanism of light. His approach is centered on the organ of sense. Since all of our senses are affected by an external stimulant, the light must have a physical explanation. He assumes that the flame must emit very tiny particles that are set in motion at great speed and when they reach our eyes they trigger our sense of sight. 

Rohault was a French gentleman who followed Descartes and popularized the systems of inquiry of nature by means of experiment and observation. He held weekly meetings at his house in Paris demonstrating physics experiments and discussing them with the audience. These meetings grew in popularity and paved the way for an enormous popular interest in physics. He had to refute Aristotellian philosophers and assure the church that they are not promoting materialism. But he was sowing the seeds of a new movement in science that continued in its pure form for several centuries.

Rohault assisted Descartes by referencing his own observations about the light and seeking all possible sources that could help him understand it a bit better. Here is an example when Rohault is tantalized by a stone that shines in the dark. “Though we have no jewels which shine in the dark, yet we have a stone that is truly luminous: This stone was accidentally found by an Italian Chymist near Boulogn in a hollow place caused by a torrent. After having put it into a fire for six hours, he took it out, and let it cool; and when it had been exposed to the light of the air for some time, upon carrying it afterwards into the dark, he first perceived it to look like a fire-coal covered over with few ashes. I have seen some shine near half a quarter of an hour, after which their light vanished, but by exposing them to the light of the air for a short time, we could make them shine again.”

Rohault thinks this is a confirmation that the stone must have some porous structure that keeps the light particles for several minutes and then pushes them back out. This would also explain the mysterious quality of the transparent objects. They have pores that allow the subtle matter of light to pass through them in straight lines. The opaque materials either don’t have these pores or the light can’t go straight all the way through them. Rohault’s system of natural philosophy was translated in Latin and English and remained an influential text book for fifty years. His theory was first overshadowed by Huygen’s theory and then published again with complimentary footnotes from Newton’s corpuscular theory.