THE SENSES. 533 



their base is reduced from G mmm. to 3 or 3.5 mmm. ; while their length 

 reaches 100 mmm., or about double what it is elsewhere. Each cone is 

 connected, through the nucleus and fibre of the preceding layer, with 

 other portions of the retina, and, no doubt, in some way, with the nerve 

 fibres of its inner layer. 



Thus, the perception of light is a process consisting of several suc- 

 cessive acts. The luminous ray passes through the tissue of the retina, 

 until it reaches the layer of rods and cones. In these elements it pro- 

 duces a reaction of whose nature we are ignorant. It might be com- 

 pared with that caused by the same agent in the sensitive film of a 

 photographic camera ; but this comparison would be only one of analogy, 

 and would not imply any identity of molecular action in the two < 

 It would simply express ihe fact, which is undoubtedly established, 

 that the luminous ray, after traversing all the other transparent and 

 refracting media of the eye without leaving any trace of its passage, 

 on arriving at the outer layers of the retina, excites in them a change 

 of condition which is the first step in the visual process. The excite- 

 ment of the retina then calls into activity the fibres of the optic n< 

 which in turn transmit the stimulus to their origin at the base of the 

 brain. Thus far, there is no conscious perception, nor even any nervous 

 effect resembling our idea of luminosity. The retina is distinguished 

 from other nervous tissues by being sensitive to light ; that is, it may 

 be thrown into a state of activity under the influence of a luminous ray. 

 But ifas no proper perception of light, any more than the silvered 

 film of''& photographic plate ; and, if the optic nerve be severed, blind- 

 ness results, -however perfect the condition of the retina. 



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

 direct action of light, are thrown into excitement by the condition of 

 the retinal tissue. There is no reason for believing that the optic 

 fibres are different in kind from those of other sensitive nerves. Their 

 office is simply that of transmitting a stimulus from and to certain 

 special structures containing nerve cells. By the optic nerve fibres 

 the stimulus is received from the retina and communicated to the brain ; 

 and the nervous centres, when thus excited, first produce the sensation 

 and perception of light. 



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

 retina, is measured by the distance between two visual rays at which 

 they can be perceived as distinct points. If the rays, coming respec- 

 tively from the top and bottom of an object, are so closely approxi- 

 mated at the retina that the two impressions are confounded, there can 

 be no distinct perception of its figure or dimensions. On the other 

 hand, if the sensibility of the retina be such that the two impressions 

 are separately perceived, the form of the object will be recognized as 

 well as its luminosity, notwithstanding the small size of its retinal 

 image. The figure of a man, six feet high, seen at a distance of ten 

 yards, makes at the cornea a visual angle of 11 30', and forms upon 

 the retina an image less than half a millimetre (J of an inch) in length ; 



