The colour of the optic disc variation with location of illumination.

The variation of the colour of the optic disc was examined with a slit lamp and contact lens in 200 normal subjects simply by moving the image of the slit across the optic disc. This event was photographed in 15 subjects and colour densitometry was performed. The temporal part of the pale central excavation changes colour to red, when illuminated nasally and vice versa. The colour change could not be provoked in subjects with normal discs with flattening of the temporal disc sector. The whole neurorim reddens the whole way round when the slit hits the optic disc border, and dependent on fundus pigmentation slight redness was produced even when the slit was placed outside the disc in the neighbourhood of the border.