The subject which Newton chose for his lectures was optics.
In optics he was the first, in 1802, to observe the dark lines in the solar spectrum.
This resolution of the original wave is the well-known "Principle of Huygens," and by its means he was enabled to prove the fundamental laws of optics, and to assign the correct construction for the direction of the extraordinary ray in uniaxial crystals.
There is a great deal of practical information on lenses in connexion with the camera and other optical instruments, and the book is valuable as a repertory of early practical optics, also for the numerous references to and extracts from previous writers.
He died on the 5th of June 1716, leaving unfinished a series of elaborate retearches on optics, and a large amount of unpublished manuscript.
