Within this peregrination, there were other movements, smaller but twice as frantic. Even by the standards of the time, Cameron was indefatigable. In Calcutta, she raised the astounding sum of fourteen thousand pounds for the relief of the Irish famine. In England, where she continued to swathe herself in Oriental silks and serve curry, there was no resisting her centripetal force. The poet Sir Henry Taylor and his wife, the Camerons’ close friends, suffered the full onslaught of Julia’s generosity: according to Taylor, “She keeps showering upon us her ‘barbaric pearls and gold,’—India shawls, turquoise bracelets, inlaid portfolios, ivory elephants, &c.” Taking your leave was no easy task—on one occasion, Julia accompanied a guest on foot to the railroad station, stirring a cup of tea along the way—and, when you were far from her, the attentions only multiplied. She once admitted to having written ninety-nine letters in a fortnight, and it was not unknown for her to race after the postman with a donkey and cart. Then, there was Dimbola, the family stronghold, in Freshwater, where the next-door neighbors were the Tennysons. Julia addressed the Poet Laureate by his first name, and vice versa. Once, when he refused to be vaccinated against smallpox, she stood at the foot of his stairs and cried, “You’re a coward, Alfred, a coward!” Anne Thackeray Ritchie, the novelist’s daughter, recalled that “Mrs. Cameron seemed to be omnipresent—organising happy things, summoning one person and another, ordering all the day and long into the night, for of an evening came impromptu plays and waltzes in the wooden ballroom, and young partners dancing under the stars.” It sounds exhausting, and one sympathizes with the visitor who, overwhelmed by the writers and painters who infested Dimbola, asked, “Is there nobody commonplace?”

In 1863, however, there was a lull. Charles was away in Ceylon, as were two of the couple’s sons. Julia was lonely (“I assume vivacity of manner for my own sake as well as for others,” she said, in a gust of candor), and one of her daughters gave her a present to keep her spirits up, adding, “It may amuse you, Mother, to try to photograph during your solitude.” The gift was a camera. She was forty-eight years old.

Being given a camera, in the eighteen-sixties, especially if you were a woman, was like being given a new Mini nowadays: the latest boxy object, practical and fun (“It may amuse you”), lending dash to your existence and allowing you to see more of your friends. What Cameron got, in fact, was two wooden boxes, one of which slid inside the other, with a French lens of fixed aperture. Images were recorded on a heavy, rectangular glass plate—the film of its day—measuring eleven inches by nine. In 1866, when the bug had bitten deep, she upgraded to an even bulkier piece of kit, which took plates of fifteen by twelve. Not only would a Mini be easier to operate. It would be easier to carry.

The mechanics of early photography may seem arduous, but they were hyper-refined when compared with the chemistry. Using the wet collodion process, which had been devised in 1851, and of which Cameron availed herself with predictable eagerness, was about as easy as running a laboratory on a waterslide, against the clock. Collodion is a flammable syrup; you poured it onto the plate, tipping this way and that to glaze the surface. One hitch with collodion is that it loses sensitivity after ten minutes, so you had to work fast. You dipped the plate in a solution of silver nitrate, and, while it was still wet, exposed it. (This was the bit we call art.) Next came development—another chemical coating, evenly applied. The plate had to be rinsed, dried, held in front of a flame “as hot as the hand will bear,” carefully varnished, washed again, and dried. To make a print from it, you submerged paper in two solutions, one of egg white, and one of silver nitrate; no enlargement was needed, so the paper was placed flat against the negative, exposed to sunlight, then washed and dried. To be fancy, you could tone it with gold chloride. Chickens also came in handy, for the eggs.

Helmut Gernsheim, whose groundbreaking 1948 survey of Cameron was inspired by seeing her work on the walls of a country railroad station, is somewhat lofty on the topic of wet collodion. “It was a great misfortune that Mrs. Cameron’s manual dexterity did not equal her artistic vision,” he writes. The technical term for this is “hooey.” There is no denying the blemishes, blots, and spots that pepper her work, but do they invalidate it, and coarsen its effect? For one thing, they are a touching reminder of the trial and error that beset any photographer of the period, especially one as headlong as Cameron. “I felt my way literally in the dark thro’ endless failures,” she stated in a letter of February, 1864. Her hands, not to mention her table linen, grew black and brown with chemicals, among them potassium cyanide, used to remove excess developer. She persevered, printing a negative that more finicky artists would have thrown away. One of her best-known images from that year, a portrait of the teen-age actress Ellen Terry, entitled “Sadness,” was patched up, rephotographed, and reissued in 1875, but I prefer the original (the J. Paul Getty Museum has a fine example), with a gaping black triangle in the lower half where the collodion peeled away from the glass. It tells us what Cameron believed was worth preserving, and what wounds could be borne in that cause. Similarly, at the Met, look at “Sappho” (1865), in which one of her housemaids, Mary Hillier, is posed in profile, wearing a richly embroidered dress, and you will witness a torn white line running from the left-hand border, imprinted by an angry crack in the plate. Do we think the less of this study in dignity, or do we see past such flaws, or through them, much as we accept them in somebody we love?

Then, there is the fuzzy matter of focus. Nothing in Cameron’s legacy is fought over with more gusto (“It is not the mission of photography to produce smudges,” one thunderous rival photographer wrote), and nothing in her own pronouncements is more abrupt than the challenge she put to Sir John Herschel in a letter: “What is focus—& who has a right to say what focus is the legitimate focus?” Herschel was highly qualified to enlighten her, being not just “an illustrious and revered as well as beloved friend” and, like his father, William, a leading astronomer but also a photographic pioneer, who discovered hypo (still used as a fixer to stabilize negatives and prints), and was the first to employ the word “negative” in this sense. But Cameron, as usual, was not expecting a reply. Scorning the “definite focus” desired by other practitioners, she preferred to stop focussing when she arrived at “something which to my eye was very beautiful,” an assertion that has encouraged later commentators to wonder about her eyesight. Even when she changed cameras and switched to a lens with a movable aperture, she chose to keep it at its widest, which meant a shallow depth of field—one thin plane of focus, with everything in front of it and behind it slipping into a haze. Factor in the lengthy exposure time, which forced Cameron’s sitters to attempt immobility—described by one of them as “torture”—and you realize how precarious the search for clarity must have been. But what exactly did she wish to make clear? Her most perceptive biographer of recent years, Victoria Olsen, gets the balance right: “Cameron could make perfectly focused images but she did not always want to.”

Herschel himself sat for Cameron, over two days in 1867. In one shot, she homed in on the most precise of focal points: the stubble on the old man’s chin. (Too wise for vanity, he said that it “beats hollow everything I ever liked in photography before.”) The Met has two more results from that sitting, very like one another, and less sharp. The rheumy eyes that have seen stars—Herschel had already published his “General Catalogue of Nebulae and Clusters of Stars”—are the lens’s target, yet, despite being viewed not through a telescope but from a few feet away, they are in a mist. Herschel is encumbered with no props; nothing gives a clue to his labors or a hint of his formal eminence. All we have is a face, emerging from blackness and staring at us with gentle perplexity, sad and unsevere, as though inquiring into the origin of our species. As Cameron wrote to a friend, “The history of the human face is a book we don’t tire of, if we can get its grand truths, & learn them by heart.” A white neckerchief encircles the sage’s throat, rhyming with the messy halo, like a solar flare, around his head. The happiest rumor surrounding this majestic photograph is that its maker prepared the way, shortly beforehand, by getting the great man to wash his hair.