A sharp flick to that fascinating toy, the kaleidoscope, will transfer a jumble of fragments into symmetrical design. To Dikon, it seemed that Mr Falls had administered just such a flick to the confused scraps of evidence that had collected about the death of Maurice Questing. If the completed pattern was not yet fully visible it was because there was some defect, not in the design but in Dikon’s faculty of observation. The central motif, the pivot of the system, was still hidden from him but it began to emerge as Falls, disregarding the sharp exclamations that broke from his listeners, and the emphatic slap with which Dr Ackrington brought his palm down on the table, went on steadily with his exposition. The affectations, and the excessive urbanity of manner, were no longer noticeable in his speech. He was grave and relentlessly methodical.

‘Now, it is a characteristic of persons afflicted with colour blindness that they are most reluctant to admit to this defect. The great Hans Gross has noted this curious attitude and says that a colourblind person, if he is forced to confess to his affliction, will behave as if guilty of some crime. Questing, when challenged by Dr Ackrington with an attempt to cause the death of Mr Smith, instead of admitting that he could not distinguish the red signal from the green, said that the signal was not working. Mr Smith told you that, later on, Questing gave him the story of the celluloid sun screen.’

‘Yes, but look here…’ Simon began and stopped short. ‘On your way,’ he said.

‘Thank you. It is also characteristic of these people that they confuse green with red and they have a predilection for that peculiar shade of pink, kaffir pink or puce as it is sometimes called, which, apparently, seems to them to be blue. A patch of red if seen by green torchlight might appear to these people to be colourless. At any rate, with the white flag removed, Questing, if colour blind, would have no standard of comparison and would most certainly expect the red flag to be white and accept it as such. Accepting for the moment my theory of Questing’s colour blindness, let us see how it squares up with the evidence of the track. Sergeant Webley,’ said Mr Falls with a slight return of his old mellifluous style, ‘will correct me if I am wrong. My own investigation took place by torchlight, remember.’