The long and unprecedented conversation I had with Policeman MacCruiskeen after I went in to him on my mission with the cigarette brought to my mind afterwards several of the more delicate speculations of de Selby, notably his investigation of the nature of time and eternity by a system of mirrors. His theory as I understand it is as follows.
If a man stands before a mirror and sees in it his reflection, what he sees is not a true reproduction of himself but a picture of himself when he was a younger man. De Selby's explanation of this phenomenon is quite simple. Light, as he points out truly enough, has an ascertained and finite rate of travel. Hence before the reflection of any object in a mirror can be said to be accomplished, it is necessary that rays of light should first strike the object and subsequently impinge on the glass, to be thrown back again to the object - to the eyes of a man, for instance. There is therefore an appreciable and calculable interval of time between the throwing by a man of a glance at his own face in a mirror and the registration of the reflected image in his eye.
So far, one may say, so good. Whether this idea is right or wrong, the amount of time involved is so negligible that few reasonable people would argue the point. But de Selby ever loath to leave well enough alone, insists on reflecting the first reflection in a further mirror and professing to detect minute changes in this second image. Ultimately he con-structed the familiar arrangement of parallel mirrors, each reflecting diminishing images of an interposed object indef-initely. The interposed object in this case was de Selby's own face and this he claims to have studied backwards through an infinity of reflections by means of 'a powerful glass'. What he states to have seen through his glass is astonishing. He claims to have noticed a growing youthfulness in the reflections of his face according as they receded, the most distant of them - too tiny to be visible to the naked eye - being the face of a beardless boy of twelve, and, to use his own words, 'a countenance of singular beauty and nobility'. He did not succeed in pursuing the matter back to the cradle 'owing to the curvature of the earth and the limitations of the telescope.'