That effect is reasoned: the light in the nave is subdued for a mystic purpose. It is not the chance result of light playing upon a fortuitous collection of coloured fragments of glass. It is a result knowingly and scientifically produced by the artist. The dim religious light which fills the threshold of the temple grows less dim as it approaches the centre of the Cross; it borrows still more transparent colours from the painter’s palette as it circles round the choir, and in the sanctuary gives place to the most lively and brilliant tones, which pour in from above. ‘What poetry is there,’ exclaims M. Ferdinand de Lasteyrie,[70] ‘in this immense gamut of tones, so cleverly arrayed. It is an admirable symbol of the light of Christianity escaping in great floods from the summit of the Cross, but throwing a lesser brillance upon those who stand afar off.’
To produce this striking effect the artist has availed himself of very simple means. In the aisles the windows are cold in tone, and the medallions,[71] as we have said, set in very broad borders. And these borders themselves, filled with a variety of patterns and ornaments, are composed of an infinite number of pieces of glass, mosaic work, the leaden frames of which contribute to the desired gloom. It is a clever application of the ‘legendary’ style, and the very choice of subjects here is in keeping with the place they occupy. The same tones prevail in the lofty windows of the nave, but there the figures are larger; huge saints and tall apostles look down from their posts upon the Cross of Christ and guard His church, S. Piat at their head; and the broad bands of colour give more access to the light. Advancing to the centre of the Cross, you perceive that the aisles are still plunged in a gloom, which is rendered yet more obscure by the unlit spaces of the great transept doors. But the roses set on high throw rainbow lights aslant the transepts, which mingle at the entrance of the choir with the mysterious tints of the nave, and the gallery of lights beneath these roses seem intended to effect the needed transition between their transparent loveliness and the opaque mass of stone beneath. In the apse, again, with its unique double aisles and the series of chapels, which form, as it were, the crown of thorns about the head of Christ upon the Cross, symbolised by the Church, there reigns a luminous obscurity. Here we have once more storied windows with medallion subjects and broad borders of topaz, emerald and ruby, rich and deep, and warmer in tone than were those of the nave. And in the centre of this sacred aureole of chapels rises the sanctuary in a blaze of light, like Jesus in the midst of His Apostles, and floods of warmly-coloured light pour down into the choir through the huge figures which fill the windows. ‘It seems,’ says M. Lasteyrie, ‘that the artist has borrowed a ray of light divine to animate his work, a ray which is brilliant at first but dies away at the entrance of the sanctuary, as if to indicate the spot in which the Christian enters into communion with his God.’