 In answer to the Papal Mandate received in July 1266, Bacon produced some very significant works. The Opus maius, Opus minus, and the related foundational work in natural philosophy, De multiplicatione specierum, the work on burning mirrors, De speculis comburentibus, together with an optical lens, were sent to the Pope c. 1267–68. They were seen as a preamble to a proposed major work on Philosophy. It remains a question as to whether the Opus tertium was sent. Together with these works in the 1260s, Bacon produced the Preface to the Works for the Pope, Communia naturalium, Communia mathematica, Epistola de secretis operibus naturae et de nullitate magiae.