Mamie Till-Mobley was adamant: “Let the people see what I’ve seen.”

In 1955, Till had just identified the body of her 14-year-old son Emmett, who had been spending the summer with his great-uncle in Money, Miss., when he was accused of flirting with a white woman. Two white men subsequently kidnapped and shot him, wrapping him in barbed wire and hanging a 75-pound fan around his neck before dumping him into the Tallahatchie River. By the time his body was recovered and returned to his home in Chicago, his face was mangled and swollen beyond recognition, with one eye hanging out of its socket.

Mamie Till-Mobley insisted on an open casket at the funeral. And she made a point of calling the African American press to photograph Emmett’s desecrated body. When Jet magazine published the image, it galvanized white and black Americans, striking what many observers consider was the first shot across the bow for the mid-century civil rights movement. “There was an extraordinary number of people in the Julian Bond-John Lewis era who were exactly Emmett Till’s age and looked at that photo and said, ‘That could be me,’ ” says Taylor Branch, author of the civil rights history “Parting the Waters.” “And four years later, they were at the sit-ins.”

Over the past several days, we’ve once again witnessed the power of an image to capture the moral imagination of a country. In many ways, the video of George Floyd dying under the knee of a white police officer has sparked the same shock, grief, anger and call to action that Mamie Till-Mobley instinctively knew would result from people bearing witness to her son’s barbaric murder.

The excruciating record of Floyd’s death emerged just days after another video had provoked a furious outcry: When Christian Cooper began recording a white woman with an unleashed dog in Central Park, he thought he was simply documenting a scofflaw; when she made a 911 call falsely accusing him of threatening her life, he kept his iPhone camera on because, he later explained to the New York Times, he refused to be “a participant in my own dehumanization.”

When Cooper’s sister Melody posted the video on Twitter, she later explained in an op-ed in the same newspaper, it was with Emmett Till’s distended face seared into her consciousness. “I wanted my brother’s calm bravery, in the face of a threatening and cowardly act, to be seen,” she wrote.

Since the groundbreaking moment in 1991 when a bystander videotaped Rodney King’s beating at the hands of Los Angeles police officers, these images — of black men and women being mistreated, disrespected, beaten, killed — have become a familiar and demoralizing element of the American visual lexicon. Before George Floyd and Christian Cooper, we saw Ahmaud Arbery gunned down while jogging two miles from his house. Before that it was Michael Brown and Eric Garner. Philando Castile. Sandra Bland. A list so long, a litany so shameful, that it has turned into its own incantatory form of elegy. Say their names. Say their names. Say their names.

Floyd’s death has inspired louder, longer protests, with the image of the knee on his neck — those endless eight minutes and 46 seconds — a painfully on-point metaphor for white supremacy, abuse of power and impunity that has stubbornly endured in America for 400 years. Could this, finally, be the image that results in something more lasting than inchoate rage? Or, in the age of cellphones, social media and decentralized activism, is it merely another data point on a timeline of trauma and despair?

In 1963, when Martin Luther King Jr., James Bevel and Fred Shuttlesworth organized more than 1,000 Birmingham students to skip school and risk arrest in what would be known as the Children’s Crusade, they saw it as their last best hope to rally much-needed national support to their campaign for legal and civic reform in that most segregated of cities. Anticipating the reaction the protest would incite, and praying that their strategy of nonviolence would hold fast, they knew that the ensuing pictures — the high-pressure water hoses turned on pigtailed girls; police officers brandishing batons with wanton malice — had the potential to jolt northerners out of their complacency and give a recalcitrant president no choice but to support desegregation.

Their instincts proved correct: When John F. Kennedy saw an Associated Press photograph of 15-year-old Walter Gadsden being bitten in the abdomen by a German shepherd held by a white policeman, he quickly sent Justice Department officials to Birmingham to intervene. Dozens upon dozens of reporters, activists, artists and clergy — white and black — descended on the city to lend their voices and bodies to the cause.

King and his fellow civil rights leaders clearly understood the power of going viral before the term was invented. For them, graphic images of racist animus and cruelty were as crucial to nonviolent protest as billy clubs and fire hoses were to segregationists.

That lesson wasn’t lost on Ava DuVernay, who talked about King’s virtuosity as an imagemaker when we spoke about her movie “Selma” in 2014. “It was not passive,” she said, describing nonviolence. “It was a tactic.”

DuVernay went on to comment that the civil rights movement, and King in particular, have been gravely misunderstood through the ensuing years. “King has been homogenized, nonviolence has been homogenized [and] smoothed over,” she said. “It’s not just about, ‘I love God and we should be peaceful.’ You can love God and be peaceful and also be strategic in the way that these things are used.”

Brenna Wynn Greer, associate professor of history at Wellesley College, agrees. “One of the most effective things about nonviolent activism is that it draws violence,” she explains. “That was the goal. . . . Those images really threw the violence of white hatred and anti-black racism into stark relief, and revealed those people and their racism to be the disturbing factors, not the African American activists.”

The George Floyd video feels like a similar tipping point. Millions of people have taken to the streets, angered not just by the inhumane way he died, but by the fact that it followed on the heels of so many other similar deaths, most recently of Arbery and Breonna Taylor, a 26-year-old emergency medical technician who was shot in her Louisville home by police executing a “no-knock” warrant. It also occurred amid a pandemic that has taken a disproportionate toll on people of color. Even white Americans who are the most deeply invested in the idea that white privilege is a myth have been forced to recognize structural, deeply embedded racism that, Greer notes, “has become so much a part of the norm that it’s invisible.”

Still, she worries that activism won’t just begin with the image, but will end there. “Because we have body cameras and social media and smartphones and everything gets circulated so much, the spectacular no longer has the effect that it used to,” Greer says. “All these videos land into this pool of images and information that provoke anger, for sure. But the images themselves have become something we can publicize as citizens and individuals and that becomes our activism. Once upon a time, they were the thing that motivated legislative change. Now, they’ve become the thing that in some ways, I worry, really stultifies change.”

Then there’s the incalculable psychic cost of the steady stream of these pictures on the people who consume them: people of color who are wounded to their core by seeing black bodies continually humiliated and destroyed, and white spectators who, even at their most well-intentioned, run the risk of internalizing messages that are profoundly dehumanizing.

“If you’re constantly seeing black people in positions of powerlessness, degradation and physical or positional inferiority, those images work on people,” Greer says. “They work on our understanding of social relationships and even equality. These images floating around showing [African Americans’] second-class status are documenting it, but my fear is also that they’re perpetuating it to some degree.”

Greer acknowledges that the video of George Floyd’s death has awakened something important among protesters who have taken to the streets, even if it’s just to deliver a primal howl of pain. And, as was demonstrated by former president Barack Obama at a virtual town hall event on Wednesday, and Rev. Al Sharpton at Floyd’s memorial service on Thursday, the ask has become more concrete, with calls for criminal justice reform and other policy changes taking on new urgency and coordinated focus. Perhaps now, sustained moral outrage can overcome the unaccountable institutional powers that have been so impervious to change.

In the meantime, like Emmett Till and the mid-century civil rights activists, Floyd’s suffering and the injustices it has exposed with such stark undeniability have managed to break down resistance that white America has historically protected at all costs.

Branch recalls King and his cohorts figuring out how to puncture the same carapace of apathy and defensiveness. They knew, he says, that “if there’s any excuse that will allow people to dodge the emotional force of what they’re doing, they will take it.”

Until they can’t.