He nodded, turning back for the door without another word. She watched him walk away for a second, watched his distinctive limp, the way his satchel snicked against his hip with each step. She remembered the first time he’d walked into the bookshop, which seemed so long ago now, and yet also as if it were only yesterday.

When she stepped outside it took Rachel a while to work out where he’d gone. He waved at her from the fence beside the forest, and she made her way across the wet grass until she could lean beside him, looking into the trees. The sound of dripping water was soothing, a symphonic fall of spent rain.

‘I think I know what happened,’ Toby said, without preamble. ‘With the fire. Or at least, I think I know how it happened.’

‘Okay,’ Rachel said slowly.

‘It’s not a happy story,’ he said.

‘I can’t imagine how it would be.’

‘Do you want to hear about it?’

‘Of course I do.’

‘All right.’ Toby took a breath, as if working out how to begin. ‘Do you remember the first time you showed me the camera obscura room?’

‘Yes, after Cullen’s funeral.’

‘I asked you if you could show me how the device worked. But the sun had set, and you told me it wouldn’t work after dark.’

‘That’s right. There’s not enough light for the lens to be able to make anything out.’

‘You said that I’d maybe be able to see the glow of the street lamps in the village, but that was it.’

‘Yes. It’s a pity, really.’

Toby smiled slightly. ‘Eveline thought so too.’

‘What do you mean?’

‘I’ve been going back through some of her diaries, and there are notes here and there – her thoughts about the build’s progress, working out the details, kinks in the plans, that sort of thing. We didn’t pay them any attention when we were first looking through the papers, as they’re just little asides, and in a lot of cases the handwriting is tiny, barely legible, even zoomed in.’

‘Right,’ Rachel said, wondering where this was going.

‘Anyway, I started to look at them more closely. The idea of seeing Braecoille after dark and how Eveline could do that was a recurring theme. One note said something like, “I wish I could see Braecoille at night. James thought the full moon might help but, alas, it made no difference. We could see nothing. We surmise it would take a far stronger, more concentrated light.”’

‘That’s funny,’ Rachel said with a frown, looking back at the tower. ‘Whenever I think of Eveline at the camera obscura, I think of her alone. I hadn’t considered that James might have gone up there with her, but of course it makes sense that he did. He knew what secret that space held.’

‘I know,’ Toby said. ‘The same thought struck me. And it’s clear to me from that note that it was a problem they discussed between them – how it might be possible to render the house visible to the camera obscura after dark. And that’s it, Rachel. That’s the key to what happened the night that Braecoille burned.’