The contributions of the orientated photosensitive and other molecules to the absorption of whole retina

Abstract

Observations have been made on the intact retinae of various animals chosen so as to give examples of golden, red and purple retinae. It is confirmed, by a method giving the absorption curve of unbleached retina, that the retinae of deep-sea fish have golden photosensitive pigments in optical densities so high that they absorb over 90% of blue-green light striking them. Despite the fact that light has passed through all the retinal layers the spectral total density curves correspond closely to those obtained on highly purified retinal extracts. After making a small correction for losses of light in the layers of retina other than the receptor layer the D_min. / D_max. ratios were about 0·25 for chrysopsin retinae and 0·25 for porphyropsin retinae. The losses of light in retinae from which the rod and cone layer has been brushed off, although varying little with wavelength, rise slowly in the near ultra-violet and red from a minimum in the yellow. The edge-fold preparation, useful for studying the dichroism of the retinal rods is described. Using such preparations it is shown that the spectral curves of dichroic difference in density are very close to those of total retinal density and therefore there can be little material absorbing light between 400 and 720 mμ orientated along the axes of unbleached rods. There is, however, such material absorbing near ultra-violet light present in unbleached retinae whose principal photosensitive pigment is derived from vitamin A₂. For both vitamin A₁ and vitamin A₂ retinae the sense of the dichroism in the near ultra-violet is reversed in the first hour following the bleaching of the photosensitive pigments. This shows that the molecules of visual white (vitamin A) in the bleached isolated retina are orientated with their axes of resonance parallel to the axes of the rods, a conclusion confirmed by studies of the polarization of the fluorescence of visual white in the bleached retina.