‘Certainly. An experiment to test Questing. It had an unfortunate result. He picked up the pipe. Quite innocently, of course, but it conveyed an unhappy impression, not only to you but to someone much more closely interested. His murderer.’

‘That’s right,’ said Simon. ‘You.’

‘But I see,’ Mr Falls continued urbanely, ‘that you are looking at this piece of yellow celluloid which I hold in my hand. I cut it off Questing’s sun screen on his car. Its colour is important. Colour, indeed, plays a significant part in our story. If you look at a red object through this celluloid it becomes a different shade of red, but it is still red. If you look at a green object through it, a similar phenomenon occurs. A bluegreen, such as one may see on a railway signal, merely becomes slightly warmer yellowish-green. If Questing said that he mistook the red signal because he saw it through this celluloid, he lied.’

‘Yes, but damn it all…’ Simon began, and got no further.

‘Questing stated that on the occasion of his almost fatal signal to Mr Smith at the railway bridge, Eru Saul wore a blue shirt, but we know that he wore a pink one. Did he lie again? That same evening he fell into Dr Ackrington’s trap, and agreed that there was no bloom on the trees at Pohutukawa Bay, when, as a matter of fact, the Bay was, and still is, scarlet with blossom. Again, did he lie? Dr Ackrington, most naturally, concluded that he had not been to the Bay, but we know now that he had. Here I should tell you, in parenthesis, that Mr Questing’s pyjamas and ties exhibit a recurrent theme of the peculiar shade of puce which it seems he did not recognize in Eru Saul’s shirt. Now we come to the final scene.’

Webley got quickly to his feet. One of the men on the verandah opened the door and came in. He, too, moved quietly. Dikon thought that only he and Gaunt had seen him and his manoeuvre. Gaunt looked quickly from this man to Webley.

‘Questing,’ said Mr Falls, ‘carried a torch when he crossed the thermal reserve. The moon was not yet up and he flashed his torchlight on the white flags that marked the path he must follow. When he reached the mound above Taupo-tapu, over which the track passes, he would see no white flags ahead of him for the one on the top had been displaced. He would, however, see the faded red flags marking the old path on which Taupo-tapu has now encroached. There are several mounds on both sides of Taupo-tapu and they look much alike by torchlight.

‘My contention is that Questing followed the red flags and so came to his death. My contention is that Questing’s murderer is the man who knew that he was colour blind.’

III