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(With 1 light-year equaling about six trillion miles, that's more than a hundred million trillion miles.) For Curtis the faint novae were bona fide proof that the nebulae resided far beyond the borders of the Milky Way. But Curtis was championing this idea too early, before the physics could explain it. Many of his fellow astronomers were still fairly skeptical, unwilling to conjure up new celestial creatures willy-nilly. For them “Occam's Razor” prevailed, the long-standing rule of thumb established by the English philosopher William of Occam in the fourteenth century. “Pluralitas non est ponenda sine necessitate,” declared Occam, which can be translated as “plurality must not be posited without necessity.” Best to choose the simplest interpretation over an unnecessarily complex one—unless forced to do otherwise. One type of nova was far more preferable than two.

Arrows point to the novae discovered by Heber Curtis

in photos of NGC-4321 taken in 1901 and 1914.

(Copyright UC Regents/Lick Observatory)

Despite the lack of support for his creative hypothesis, Curtis was still gaining appreciable momentum on his endeavor, at least until World War I intervened. Just months after the United States officially joined the fight in 1917, Curtis went first to San Diego and then to Berkeley to teach officer recruits navigation. Afterward, he proceeded to Washington, D.C., to work for the Bureau of Standards on the design and development of military optical devices. Before taking his leave, though, Curtis made sure to compile a master list of the spiral nebulae he had photographed with the Crossley, by now more than five hundred. And as before, each of his photographs revealed ever more nebulae, pale and murky, surrounding the more consequential spirals that he officially cataloged. On one plate alone he counted 304 additional spirals. Keeler had estimated that 120,000 spiral nebulae were within observational range of the Crossley. Another Lick astronomer later upped that number to 500,000. Now Curtis was raising the figure even higher. “The great numbers of small spirals found on nearly all my plates of regions distant from the Milky Way, long since led me to the belief that [an earlier] estimate of half a million was likely to be under, rather than in excess of, the truth,” he reported. “[I] believe that the total number accessible with the Crossley Reflector with rapid plates and exposures of from two to three hours may well exceed 1,000,000.” This was an astounding hike in the spiral estimate.

While still in the U.S. capital wrapping up his work after the 1918 armistice, Curtis was invited to deliver a semipopular lecture on the spiral nebulae before the Washington Academy of Sciences and the Philosophical Society of Washington. His expertise on the topic was getting noticed. “Get up a collection of about 40 classy slides and send to me at once,” he wrote Campbell at Lick in great excitement. Curtis was thrilled at the opportunity, his first actually, to lay out before an influential scientific conclave all his hard-won evidence in support of the island-universe theory. He planned to use a lantern slide—the early-twentieth-century version of PowerPoint—to display the various types of spirals he had come across, to point out the dark lanes running through them, and to reveal the many fainter nebulae lurking in the background of his photographs of the spirals.

On the appointed day—March 15, 1919—a large audience gathered to hear Curtis in the new lecture room at Washington's prestigious Cosmos Club (then located at Lafayette Square), the traditional meeting place for the city's intelligentsia. Curtis opened with a tip of the hat to William Herschel. “The history of scientific discovery affords many instances where men with some strange gift of intuition have looked ahead from meager data, and have glimpsed or guessed truths which have been fully verified only after the lapse of decades or centuries,” he said. “We have now, as far as the spiral nebulae are concerned, come back to the standpoint of Herschel's fortunate, though