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Hooke: "A single votary, Mr. Leeuwenhoek"
In February 1692, when as it turns out Leeuwenhoek was not halfway through his long career, Robert Hooke delivered a lecture about the history and future of both the telescope and microscope.
Much the same has been the Fate of microscopes, as to their Inventions, Improvements, Use, Neglect and Slighting, which are now reduced almost to a single votary, which is Mr. Leeuwenhoek;
besides whom, I hear of none that make any other Use of that Instrument, but for Diversion and Pastime, and that by reason it is become a portable Instrument, and easy to be carried in one's Pocket.
At the time, the astronomer Edmond Halley, ally of Isaac Newton, had been editor of Philosophical Transactions for volume 16, and Leeuwenhoek had not had a letter published in seven years, since 1685. Halley and Newton had little use for microscopes and they were long-time opponents of Hooke, who was Leeuwenhoek's chief proponent at the Society. Hooke continued:
If we enquire into the Reason of this Change of Humour, in Men of Learning, in so short a time, we shall find that most of those, who formerly promoted these Enquiries, are gone off the Stage; and with the present Generation of Men the Opinion prevails, that the Subjects to be enquired into are exhausted, and no more is to be done:
Besides they pretend that all the Discoveries that have been hitherto, or that can be made, for the future by these Instruments will afford no gainful profit, and all other Notions are insipid with them, besides such as bring ready Money.
No more to be done? All the other contemporary microscopists studied the microscopic structure of macro organisms, as did Leeuwenhoek at times, and perhaps those subjects appeared to be "exhausted". Leeuwenhoek alone discovered the living world of microorganisms, where the subjects were far from exhausted.
When Hooke gave his lecture in 1692 (though not published until 1726), Halley was about to be replaced as editor of Philosophical Transactions by Richard Waller and then Hans Sloane (right), who would again begin publishing Leeuwenhoek. Indeed, as Hooke predicted, there was much more to be done:
But those, who make such estimates, may, perhaps, find themselves very much mistaken in their Judgment, if the Subjects are duly prosecuted, as they are capable of so being.
For, as to the Discoveries that may be made in both Kinds, I conceive they are vastly greater, both for Number and Value, than those few that have been already made; and not only for the Information of the Intellect, but what answers their greater Objection, even for the increasing their Treasure.
While he was out of favor with editor Halley during those years, Leeuwenhoek wrote dozens of letters, most of which he sent to the Society and by all accounts were not even acknowledged by Halley. Perhaps concluding that his publishing career in London was finished, Leeuwenhoek published the letters himself in parallel Dutch and Latin editions.
Hooke concluded that the microscope:
is become almost out of Use and Repute: So that Mr. Leeuwenhoek seems to be the principal Person left that cultivates those Enquiries. Which is not for Want of Considerable Materials to be discover'd but for Want of the inquisitive Genius of the present Age.
In fact, it would take more than a hundred years before the compound microscope would reduce the problems of spherical and chromatic aberration and catch up to Leeuwenhoek's in magnifying power and resolution. It would be even longer than that before scientists could put Leeuwenhoek's microbiological discoveries into a context. They began re-discovering Leeuwenhoek's protozoa and bacteria, related them in a Linnaean taxonomy, gave them names, and began understanding how Leeuwenhoek's "little animals" enabled human life and occasionally threatened it, i.e., germs.
Unlike his contemporary Vermeer, whose death at an early age meant the loss of all the paintings he never lived to paint, Leeuwenhoek discovered only things that were already there. Had he died early, everything that he discovered would have been discovered by someone else, though perhaps not for 200 years.
So Leeuwenhoek's accomplishment, sustained for half a century, is as much an individual accomplishment as a scientific one. He had the leisure, the motivation, and the habits of mind that he needed. He was tenacious and honest, and he had an eye for the important detail. If he had a "secret", perhaps that was it.