The Professors of Light and Darkness were the joint deans of the School of Light and Darkness in Sanctaphrax. The Professor of Light was in charge of light-studies, and the Professor of Darkness was in charge of darkness-studies.
The Old Professors of Light and Darkness
The Professors of Light and Darkness during the Quint and Twig trilogies (whose real names were never revealed) met each other at an early age and became close friends, drawn together by their shared fascination of luminosity. So strong was their bond that they began to mirror each other's actions and personalities, even assuming each other's physical characteristics.
Linius Pallitax, the Most High Academe of Sanctaphrax, elected them both to the position of Next-Most High Academe, and they assumed many of the ceremonial duties of the great floating city, such as inspecting the stormphrax in the Treasury once every year on Treasury Day.
At the beginning of The Winter Knights, the Professors of Light and Darkness became joint Most High Academes, after the death of Linius Pallitax. Later on, Vilnix Pompolnius convinced the academics of Sanctaphrax that the Professors of Light and Darkness were inept, and persuaded them to elect him Most High Academe in their place.
The Professor of Light accompanied Cloud Wolf on a stormchasing voyage, but broke his neck while abandoning ship in the Twilight Woods. Because of the Twilight Woods' effects, the Professor of Light neither died nor felt any pain. However, his health began to deteriorate once they left the woods. When the surviving crew members encountered Screed Toe-Taker in the Mire, the Professor of Light discovered the remains of the Windcutter. Recognising it as the ship of Screedius Tollinix, he called out the name and caused Screed to let his guard down and be killed. The professor was not aware that Screed and Screedius were, in fact, one and the same. His life almost at an end, the Professor of Light told Twig the elusive method of safe phraxdust production, and asked him to relay it to his friend, the Professor of Darkness. As the birds were starting to head towards the newly deceased body, Twig let the professor sink below the surface of the Mire, remarking that this was not the final resting place he deserved.
After Twig stabilized Sanctaphrax with fresh supplies of Stormphrax, Vilnix Pompolnius was dethroned, and the Professor of Darkness resumed his duties as Most High Academe. From then on, the Professor of Darkness would ring a bell at the precise moment of twilight, allowing the residents of Undertown to produce their own phraxdust. Twig and his crew left aboard the Edgedancer to find Cloud Wolf, who had been lost in Open Sky. At some point after this, during a particularly violent storm, the Professor of Darkness observed eight shooting stars soaring across the sky. He was informed by Kraan that one of these had landed in the Stone Gardens. He investigated, and found that this shooting star was none other than Twig, who would not speak. Taking Twig to Sanctaphrax, he made him the new Sub-Professor of Light. When Twig was informed by Cowlquape that there had been more shooting stars, Twig opted to leave Sanctaphrax, as there was a chance the other members of his crew were still alive. The Professor of Darkness wearily accepted Twig's decision, knowing that Twig was a Sky Pirate more than he was a professor. When the Mother Storm arrived and Sanctaphrax had to be unchained, the professor, unwilling to let go of the great floating city, seized the severed Anchor Chain and floated off into Open Sky.
The New Professors of Light and Darkness
During the Rook trilogy, the new Professors of Light and Darkness were Ulbus Vespius and Tallus Penitax. These two scholars fled the Tower of Night when the schemes of Orbix Xaxis became too evil for their tastes, and joined the Librarians Academic. Later on, they joined the council of the Free Glades when the Librarians relocated there.