IntroductionThe prehistory of the mysterious milky patches in the sky known as nebulae goes back to Antiquity, but eighteenth-century writers often took their lead from the short paper that Edmond Halley published in 1715, An account of several nebulae or like clouds.1 Halley listed six nebulae: those known to us as Ml 1, M 13, M22, M31 and M42, together with ? Centauri which too far south to be visible from England. They are, Halley declared, Light coming from an extraordinary great Space in the Ether; through which Medium diffused, that shines with its own proper Lustre.But star system so distant that existing telescopes were not powerful enough to 'resolve' it into stars would also appear nebulous, and there could be little doubt that such systems existed. The question, therefore, was whether all nebulae were simply star clusters disguised by distance, or whether Halley was right and some were formed of lucid medium, or nebulosity as William Herschel (1738-1822) preferred to call it.Herschel was introduced to astronomy in 1772-73 by two authors, both of whom drew on Halley's paper. What they had to say about Halley's immediately captured his attention. Herschel's own theorizing on nebulae falls into three periods. For decade, 1774-84, Herschel had good reason to believe in true nebulosity; then, in 1784, new observations led him to reverse his opinion and he now equated nebulae with star clusters - only in 1790 to come across evidence that compelled him once more to accept the existence of true nebulosity. Here we investigate in detail the first of these periods, while in Part 2 we study the second and third.1774-1781: Does the Orion Nebula Alter Shape?A knowledge of the construction of the heavens has always been the ultimate object of my observations, wrote William Herschel in 181 1 when commencing his final series of papers on this subject.2 Nearly four decades earlier, when Herschel first developed his passion for astronomy and in particular his fascination for cosmology, he was one of the two leading musicians in the fashionable spa resort of Bath in the west of England.3 As an astronomer he was entirely self-taught, learning his telescope-building from the formidable two volumes on Opticks by the Cambridge professor Robert Smith,4 and his astronomy from the popular Astronomy by the former shepherd boy, James Ferguson.5Smith began his brief chapter on Telescopical discoveries in the Fixt by informing his readers that the Milky Way wasnothing else but prodigious number of very minute stars, so close to one another that the naked eye can only perceive whitish mixture of their faint lights. This was Galileo's discovery, who found also that those faint stars, which Astronomers call Nebulosae, appeared through his telescope to be small clusters of very minute stars.6In 1656 Christiaan Huygens had seen a whitish cloud in Orion's Sword, and Smith reproduced Huygens's sketch. According to Huygens, he tells readers,the three little stars very near one another (marked ? by Bayer), together with four more, shone out as it were through whitish cloud, much brighter than the ambient sky: which being very black and serene caused that part to appear like an aperture, that gave prospect into brighter region.7An article in Philosophical transactions in 1715, Smith continued, had reported five more of these lucid spots, where it seemed that there was perpetual uninterrupted day.Ferguson had more to say. He lists five of these six lucid spots under this same heading; of the Andromeda Nebula, M31, he says intriguingly that it is sometimes invisible, in which case it could not be vast star system.8 The sixth, the Orion Nebula, he lists under separate heading, Cloudy stars. These look like dim Stars to the naked eye; but through telescope they appear broad illuminated parts of the Sky; in some of which one Star, in others more. …