86 



ELECTROMAGNETIC RADIATIONS AND MATTER 



a center called the fovea centralis. The distribution of rod cells is different 

 (Figure 4-8) — practically none at the fovea, but otherwise distributed in 

 great numbers over the whole area of the retina. 



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HORIZONTAL ANGLE (DEGREES) FROM FOVEA CENTRALIS 



Figure 4-8. The amount of rhodopsin and the number of rods per unit area 

 have a similar dependence on angle bounded by the incoming light and the 

 central meridian in which incoming light falls directly on the fovea. The optic 

 disc, where the optic nerve enters, is about 16° to the nasal side, and there- 

 fore a blind spot exists there. (Locate the blind spot in your right eye by first 

 focusing the eye on the black dot, then turning the eye 16° to the left — i.e., 

 about 4 in. if the dot is 10 in. from the eye.) (After Rushton.') 



A brief discussion is now given of those molecules, known as pigments, 

 which are not only the absorbers of the incoming radiation but also the 

 transducers, the "machines" by which the incoming energy is trapped and 

 "led across" into another form, not heat, which can trigger the optic nerve. 

 Actually there are two separate subjects to discuss: twilight vision and color 

 vision. Although much has been learned by direct experiment on animals, 

 Rushton 1 complained in his recent review: "Measurements upon human pig- 

 ments have only just begun, and it is to be hoped that far better experiments 

 will be made." We give here a summary of the present understanding of this 



