VISION. 239 



the bacillary layer are entirely replaced by tlie more highly 

 organized cones. It will be noticed that when the eye is 

 fixed on the cross, not only is that mark the most distinctly 

 visible object, but letters as far away from it as the position 

 of the round ball, although still a long distance removed 

 from the circumference of the field of vision, are so vaguely 

 seen that they cannot be distinguished when the ball is hid 

 from view, even though the figure is so arranged that this 

 happens when the book is held at a good focal distance. But 

 the optic pore, it will be recollected, is only one-tenth of an 

 inch' from the axis of the eye, therefore this observation 

 shows that the retina, at the circumference of a circle with 

 a radius of a tenth of an inch, and the axis of the eye as its 

 centre, has much less acute sensibility than the yellow spot. 

 And inasmuch as there is no interruption of the outline of 

 the field of vision corresponding to the position of the optic 

 pore, but the place where such an interruption might be 

 expected to show itself is filled up with the appearance round 

 about, whether light, dark, or coloured, there is evidence that 

 the sensation originating in each cone or rod is not strictly 

 limited; although in other parts of the retina it is practically 

 limited by the sensations from the rods round about. This 

 explains the spreading of the appearance of light round exces- 

 sively luminous objects. 



It being clear that the layer of nerve fibres is not the part 

 t)f the retina in which the rays of light produce nervous 

 impression, it is curious to observe that the rays have to 

 pierce these fibres and the strata of the retina before reach- 

 ing the rods and cones on which they act. This shows that 

 the susceptibility to a stimulus so fine as light depends not 

 on the nervous structure but on the peculiar terminal organ 

 added thereto. 



176. The most important of the structures through which 

 the rays of light pass on their way to the retina is the crystal- 

 line lens. It is comparable with a biconvex lens of glass 

 made by an optician. The rays of light from each visible point 

 in the landscape pass through the whole aperture of the pupil, 

 and are refracted towards one another as they enter the 

 anterior convex surface of the dense substance of the lens, 

 and again as they emerge from its convex surface behind. 



