INTRODUCTION AND METHODS 



specific *luminosity' mechanisms (the dominators) while the light- 

 adapted retina possesses, in addition, mechanisms (the modulators) 

 which may act as mediators of colour vision. 



THE VISUAL PIGMENTS 



The chain of events which results when Hght falls on the retina has 

 its origin in the rod-cone layer. How does this layer respond to hght 

 and thus initiate the electrical changes which eventually reach the 

 brain? The first requirement for any of the various mechanisms 

 which might be proposed is that the light shall be absorbed, for it is 

 only so that its energy can be utihzed. One would expect to find, 

 therefore, that the rods and cones were associated with Hght- 

 absorbing substances. 



Such is, in fact, the case; coloured substances — the visual pig- 

 ments — can be extracted from the rod-cone layer of retinae. It is 

 now generally accepted that the spectral regions in which the sensory 

 mechanisms of vision are active are determined by the light-absorbing 

 properties of these substances. Indeed, a few satisfactory correla- 

 tions have been made. The best-known example is the agreement 

 between the sensitivity curve for the dark-adapted human eye and 

 the absorption spectrum of human 'visual purple.' A comparison of 

 modern data for these is shown in Fig. 1.3. 



It is not the purpose of this book to deal with the visual aspects of 

 the visual pigments, however, but to describe their physical and 

 chemical properties and the methods which have been used to 

 determine them. 



PREPARATION OF VISUAL PIGMENT SOLUTIONS 



The visual pigments are highly photosensitive : if they are exposed 

 to such light as they can absorb, they are destroyed. Consequently, 

 when one is carrying out extraction procedures, it is necessary to 

 work under 'photographic' conditions, i.e. in a dark room lit by 

 non-actinic Hght. Generally, a deep red safe-Hght is used. For 

 certain operations, such as removal of the retinae, a more locafized 

 illumination may be needed. This can be safely provided by a small 

 torch fitted with Ilford 'monochromatic' filter No. 609, which 

 transmits from 650 m/f to the infra-red. 



The first step in the preparation of a visual pigment solution is to 

 dark-adapt the animal. During the darkness period (which need be 



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