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Molecular Basis of Vision 



I. Color Vision and Photopigments 



The phenomena of vision form the basis for Chapters 2 and 7. In these 

 chapters, the eye was considered as an optical system which focuses 

 images onto the retina. The retina was shown to act as a transducer 

 converting the light to neural impulses. These, in turn, appear to be 

 sorted and analyzed both within the retina and within the brain to give 

 rise finally to the sensation of vision. 



Part of the visual sensation consists in recognizing different colors. 

 The color sensed is a function of the wavelength of the incident light ; the 

 complexity of this function is emphasized by the experiments described 

 in Chapter 7. Nevertheless, any discrimination of different hues is due 

 to the presence of receptors which selectively absorb light energy in 

 certain wavelength regions. The photosensitive pigments are altered 

 by the photons absorbed. These changes take place on a molecular level 

 and are far too small to be revealed by any histological method. This 

 particular aspect of vision is, therefore, in the realm of molecular 

 biology. 



All physiological or psychophysical experiments indicate that there 

 must be at least three pigments within the retina of mammals. Whereas 

 some experiments indicate that there may be many more than this, 



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