792 PHYSIOLOGY. 



Using the above values, the positions of the cardinal points of 

 Gauss, of the human eye on the optical axis, calculated from the 

 apex of the cornea,, are as follows: 



The first principal focus is situated 13.745 millimeters in front 

 of the cornea. 



The other points are behind the cornea: 



The first principal point 1.7532 mm. 



The second principal point 2.1101 mm. 



The difference is 0.3569 mm. 



The first nodal point 6.9685 mm. 



The second nodal point 7.3254 mm. 



The second principal focus 2.28237 mm. 



These values are shown in Fig. 340, but are three times as great 

 as in nature. 



From these data are shown the course of rays through the eye 

 and the position and size of images. 



Perception of Light. 



Light is due to vibrations of ether; a proper conception of them 

 gives the sensation of sight. Transmission of light, with air as a 

 medium, is 186,000 miles per second. The rapidity of the vibra- 

 tions influences the sensation produced, for color is for luminous 

 sensation what height is for sound. The inferior limit of visible 

 vibrations is represented by the color red; the superior limit is 

 exemplified in violet. 



For light to be perceived physiologically by any individual it 

 must make an impression upon the retina. The light falling upon 

 the retina immediately stirs up certain changes in it which in turn 

 give rise to nervous changes in the fibers of the optic nerve. This 

 last change, or "visual impulse," produces a further series of events 

 within the brain, one effect of which is a change in our conscious- 

 ness; that is, there is a sensation. 



The point upon the retina at which the impressions are strongest 

 and most exact is the macula lutea and its fovea centralis. The 

 anatomical layer designed to be impinged upon by a distinct image 

 is the membrane of Jacobson, the layer of rods and cones. As only 

 the cones, and no rods, are found in the fovea centralis, it is the 

 point where objects are fixed. Hence it must be held that the cones 

 are the specific elements of the retina that are designed to make the 

 individual perceive a luminous impression precisely. Nevertheless, 

 the field of vision, though indistinct toward its periphery, is very 

 much enlarged. 



