The work is a "vade mecum" of the experimenter's art, displaying in many examples how to use observation to propose factual generalisations about the physical world and then exclude competing explanations by specific experimental tests. However, unlike the "Principia", which vowed "Non fingo hypotheses" or "I make no hypotheses" outside the deductive method, the "Opticks" develops conjectures about light that go beyond the experimental evidence: for example, that the physical behaviour of light was due its "corpuscular" nature as small particles, or that perceived colours were harmonically proportioned like the tones of a diatonic musical scale.