THE VISUAL PIGMENTS 



OBSERVATIONS OF VISUAL PIGMENTS IN 

 LIVING RETINAE 



In certain cases it is possible to see the visual pigment in a living 

 animal's eye. Thus, simply by looking through an ophthalmoscope 

 into the eyes of crocodiles and certain fish which have white tapeta, 

 ABELSDORFF (1897, 1898) had observed the bleaching of a purple 

 pigment. 



Direct visual observation of this sort depends on there being a 

 substantial reflection of light from the post-retinal structures. The 

 colour of a piece of glass may not be readily seen when it is placed on 

 a dark surface, but is obvious when laid on white paper. Similarly 

 the presence of a white tapetum behind the retina reveals the retinal 

 colour to the observer. 



In other animals where reflection from the tapetum is not neutral 

 or where, as in man, there is no tapetum, the retinal colour cannot be 

 directly observed. Nevertheless, even in these cases, some hght is 

 reflected from post-retinal surfaces and so out of the eye. This Ught 

 has passed twice through the retina — once after entry into the eye 

 and again after reflection. It thus bears an *imprint' of the retinal 

 absorption spectrum; albeit an imprint distorted by the effects of 

 non-neutral reflection from the fundus or tapetum, and of absorption 

 in the pre-retinal tissues. But if the characteristics of the light were 

 determined for an eye both before and after the visual pigment of its 

 retina had been bleached, the changes should give a measure of the 

 difference spectrum of the visual pigment alone. 



In 1952, BRiNDLEY and willmer applied the ophthalmoscope 

 principle of abelsdorff in a quantitative sense to the human eye. 

 The light from a monochromator was divided so that part entered 

 the subject's eye and the rest traversed a comparison pathway which 

 included a neutral wedge filter. By a suitable adjustment of the 

 wedge, the intensity of the comparison beam was matched with that 

 reflected from the fundus of the subject's eye. This was done for 

 several wavelength settings of the monochromator. By comparing 

 the reflection of light from the macular with that from a peripheral 

 area of the retina, brindley and willmer estimated the density of 

 macular pigmentation. They also attempted, with partial success, to 

 detect the bleaching of visual purple in the dark-adapted periphery 

 of the retina. 



Another investigation which indirectly stimulated in vivo studies 



194 



