VISION 



337 



FIG. 190 



of accommodating its function not only to the light's brightness, but to 

 the distance or size of the object. Vibrations in the hypothetical "ether" 

 of space occurring between 392,000,000,000,000 and 757,000,000,000,000 

 times per second give the eyes their proper stimulus. Vibrations 

 outside this narrow range we do not perceive as light, but as heat, 

 electricity, or other manifestations of vibratile energy. The tendency 

 now is to think of this ether as attenuated matter streaming in radia- 

 tions from the sun. 



(The common refractive defects of human vision and their remedies 

 with lenses will be found described with sufficient detail for our purpose 

 in the Appendix, page 526, etc.) 



The Receptive Apparatus of the Eye (as distinct from the transmitting 

 mechanism) consists of the retina and the optic nerve and tracts and the 

 centers behind them. Of all the complex 

 structures of the retina (not yet much 

 understood), the visual cells are the essen- 

 tial portions. These are the rods and the 

 cones. Each rod-cell consists of a rod 

 and a rod-fiber with its nucleus. The 

 rod, averaging 45 microns long, is made 

 up of two segments, the outer of which 

 (doubly refracting) consists of numerous 

 transverse disks, while the inner segment 

 is striated longitudinally. The rod-fibers 

 extend downward to the outer nuclear layer 

 (see Fig. 190) where they end in tiny 

 bulbs. The rods contain in their outer 

 parts the essential pigment called visual 

 purple or rhodopsin, and granules in their 

 inner ends. The cone-cells consist each 

 of a cone and a nucleated cone-fiber. 

 The cone averages 20 microns long, and 

 its fiber, like that of the rod, extends in- 

 ward to the outer nuclear layer of the 

 retina where it ends in a branched basal 

 plate. The proportion of rods to cones 

 varies much in the different retinal regions. 

 In the macula lutea, the sole region of 

 sharply focused vision, there are no rods, 

 but in other parts an average of fourteen 

 rods to one cone are found, but fewer cones the farther away from 

 the macula a region lies. It is noteworthy that the cones contain 

 no visual purple. The rods and cones are the only ocular elements 

 capable of reacting to the minute vibrations of the luminiferous 

 ether, but how they do so is quite unknown evidently, however, by a 

 transformation of force, the light waves becoming waves of nervous 

 impulse. About seven cones, a hundred rods, and seven of the 

 22 



Different shapes of retinal rods: A, 

 that of the perch; B, man; C, pig; 

 D, green rod of the frog; E, red rod 

 of the frog; a, external segment; b, 

 internal segment; c, cellular body. 

 (Greeff.) 



