626 THE SENSES. 



On the other hand, the optic nerve fibres, which are insensible to light 

 itself, are thrown into excitement by the changed condition of the retinal 

 tissue. There is no reason for believing that the action of the fibres 

 of the optic nerve is different in kind from that of other sensitive nerve 

 fibres. Their office is simply that of receiving and communicating a 

 stimulus from and to certain special structures containing nerve cells. 

 In the case of the optic nerve, the stimulus is received from the retina 

 and communicated to the nervous centres of the brain. These nervous 

 centres, when excited by the stimulus thus received, first produce the 

 phenomenon of the perception of light. The preceding nervous actions, 

 in the retina and optic nerve, though necessary to the final result, have 

 no direct connection with consciousness. The conscious perception of 

 light and of luminous objects is the last step in the process of vision, 

 and is effected by a special act of the gray matter of the brain. 



Acuteness of Vision in the Retina. The acuteness of vision, so far 

 as it is connected with the sensibility of the retina, depends upon the 

 minimum distance from each other of two visual rays, at which they 

 can still be perceived as distinct points. If the luminous rays, coming 

 respectively from the top and bottom of an object, are so closely ap- 

 proximated, where they strike the retina, that the two impressions are 

 confounded, there can be no distinct perception of its figure or dimen- 

 sions. On the other hand, if the sensibility of the retina be such that 

 the two impressions are still perceived as separate from each other, the 

 form of the object will be recognized as well as its luminosity, notwith- 

 standing the small size of its retinal image. The figure of a man, six 

 feet high, seen at the distance of ten yards, makes at the cornea a visual 

 angle of 11 30', and forms upon the retina an image which is less than 

 half a millimetre (^ of an inch) in length ; and yet an abundance of 

 details are distinctly perceptible within this space. The extreme limit 

 of approximation at which two points may be distinguished from each 

 other has been examined by the observation of fixed stars, and by that 

 of parallel threads of the spider's web, or of fine metallic wires, placed 

 at known distances from each other. 1 The general result of these 

 examinations has shown that, for the average of well-formed eyes, the 

 smallest visual angle, at which two adjacent points or lines can be dis- 

 tinguished, is from 60 to 13 seconds ; corresponding to a distance upon 

 the retina of from 4 to 5 rnmm. According to the measurements of 

 Schultze, the diameter of the retinal cones, at the fovea centralis, is 

 from 3 to 3.5 mmm. ; and if two points of light were separated at the 

 retina by a less distance than this, they would often fall upon the same 

 cone, and consequently excite the same nucleus and fibre in the adjacent 

 layer. If the diameter of the cones be the element which determines 

 the limit of acuteness of vision, two luminous points, to be distinctly 

 perceptible, must be separated upon the retina by a distance of at least 

 3 mmm., and must have a visual angle with each other of at least 42 



1 Helmholtz, Optique Physiologique. Paris, 1867, p. 292. 



