Four days later, the mailman brought something else — a box of chocolates and a letter addressed to Miss Elsie Wright.

“I am myself convinced of fairies — have been all my life, but the best photograph I have ever seen is undoubtedly yours,” Mr. Gardner wrote.

The subject is quite an important one and you would be doing us all a considerable service if you could get further photographs of a similar kind of the various varieties. I will most willingly assist in every way possible. . . . Many of us wish to learn more about this delightful side of life and if you can help I shall be immensely grateful. Will you think of it and let me know?

It was Elsie’s mother who wrote Mr. Gardner back.

She could have explained that Elsie was a terrible speller who hardly ever wrote letters. But Elsie’s mother was nice enough not to.

Instead, she said that she was writing for her daughter because Elsie was sick:

“She says I must thank you ever so many times, indeed it was very kind of you, but she does not know if she can take any more,” Elsie’s mother wrote.

The little niece of mine from South Africa, the one on the picture [h]as gone away. If she does take any more I will send them on. Thank you very much for the offer of the camera but we have two, and her dad lets her have the use of them now.

Yours Sincerely,

Polly Wright

Now what?

Mr. Gardner had sent a box of chocolates and a letter for Elsie, not her mother.

He did not seem to be going away.

And far away in London, the fairy machine was rolling onward.

In London, Mr. Gardner had had lantern slides made from the two glass plates. Now he was showing the pictures of Frances and the fairies and Elsie and the gnome on screens at auditoriums all over town. Every day, more and more people heard about the fairy photographs.

One of them was a writer who was conducting his own research on fairies. He planned to publish an article — with firsthand accounts of actual fairy sightings — in one of England’s most popular illustrated magazines, the Strand.

And they did make fascinating reading.

In one account, a man called Mr. Lonsdale described the fairies that he and his friend Mr. Turvey had seen in Mr. Turvey’s garden.

We sat in a hut which had an open front looking on to the lawn. We had been perfectly quiet for some time, neither talking nor moving, as was often our habit. Suddenly I was conscious of a movement on the edge of the lawn, which on that side went up to a grove of pine trees. Looking closely, I saw several little figures dressed in brown peering through the bushes. They remained quiet for a few minutes and then disappeared. In a few seconds a dozen or more small people, about two feet in height, in bright clothes and with radiant faces, ran on to the lawn, dancing hither and thither. I glanced at Turvey to see if he saw anything, and whispered, “Do you see them?” He nodded. These fairies played about, gradually approaching the hut. One little fellow, bolder than the others, came to a croquet hoop close to the hut and, using the hoop as a horizontal bar, turned round and round it, much to our amusement. Some of the others watched him, while others danced about, not in any set dance, but seemingly moving in sheer joy. This continued for four or five minutes, when suddenly, evidently in response to some signal or warning from those dressed in brown, who had remained at the edge of the lawn, they all ran into the wood. Just then a maid appeared coming from the house with tea. Never was tea so unwelcome, as evidently its appearance was the cause of the disappearance of our little visitors.

The writer took stories like these perfectly seriously, even though he knew that people might make fun of him for doing so. “What does it matter what anyone says of me,” he once said to his mother in a letter. “I have a good hide by this time.”

He did have a good hide. He had to, for he was famous all over England, and even in America and Australia. His name was Sir Arthur Conan Doyle.

Most people knew him as the creator of Sherlock Holmes, the world’s most famous detective.

Sherlock Holmes was a keen-eyed, hawk-nosed man who had made detective work into a precise and rational science. Sherlock Holmes could put the tiniest clues together to find the truth. He was almost impossible to fool.

So it might seem surprising that his creator, Sir Arthur, believed in fairies. But he did.

To Sir Arthur, fairies were part of a spirit world that coexisted with the everyday world he saw all around him. The spirit world was invisible, though. Only special people could see it or hear the voices of the spirits who lived in it. Those spirits included the ghosts of dead people, Sir Arthur believed. His own son, who had died of sickness after being wounded in the Great War, was one of them.

These days, Sir Arthur went to what were called séances, where he believed people called mediums might be able to give him messages from his son in the spirit world.

Sir Arthur had many friends who, like him, were interested in the invisible world. One day, Sir Arthur happened to be talking to a friend who asked him, had he heard the talk about some actual photographs of fairies? The friend hadn’t actually seen them, but he had a friend who might know about it.

Sir Arthur talked to the friend, and then wrote a letter to a friend of that friend, and so on. He followed the trail of friends and relations until someone gave him Mr. Gardner’s name.

When the two men met, Sir Arthur was relieved to see that Mr. Gardner seemed quite rational and respectable — not wild-eyed at all.

Sir Arthur and Mr. Gardner agreed that, together, they would conduct a step-by-step investigation of the matter of the fairy photographs.

Sir Arthur would handle the London end of the detective work.

Then the plan was that Mr. Gardner would take the train up to Cottingley, visit Elsie and her family, and see what kind of people they were.

If there was any sort of fraud involved, the two men thought, it would be best to uncover it right away.

Sir Arthur went to his men’s club, the Athenaeum, and showed the fairy pictures to a friend of his, Sir Oliver Lodge, an expert in “psychic matters.” Sir Oliver was skeptical: he suspected the ring of dancers had been somehow superimposed on a different background.

Sir Arthur didn’t agree. “I argued that we had certainly traced the pictures to two children of the artisan [working] class, and that such photographic tricks would be entirely beyond them,” he wrote. Working-class children, surely, would not be able to pull off such a sophisticated trick.

Sir Arthur also took the two glass plates to Kingsway, a broad London boulevard where an American firm, the George Eastman Kodak Company, had built one of London’s most modern buildings. There, Sir Arthur talked with two of Kodak’s experts. “They examined the plates carefully, and neither of them could find any evidence of superposition, or other trick,” Sir Arthur wrote.

The experts were not convinced the fairies were real — they thought that if a person had the right resources and knowledge, it was certainly possible to fake them.

Sir Arthur thought about this.

That meant that the only way to tell if the photographs were fake was to find out more about the girls who had taken them. Were they honest? Were they open?

What kind of girls were they?