THE PEKCEPTION OF LIGHT. 15 



way, on account of the difficulty of examination of this 

 pulpy structure, it is pretty certain that they join on 

 to those nerve-fibres which line the front surface of the 

 retina, and so pass on, through the optic nerve, to the 

 brain. When I say " pass on " I mean of course as you 

 trace them along ; there is no motion in the case. This is a 

 very remarkable structure. Has it any object ? What is its 

 object? Now we know by experience that if we have a 

 single point of light exposed to us, the impression is that of 

 a single point of light in the field of view. If there be two 

 such points we have the impression of two luminous points 

 occupying different positions in the field of view. Now two 

 such points may be very close to one another, and yet we still 

 see them as two. It is found that the limit of closeness, 

 beyond which we are unable to distinguish two objects as two, 

 is such that a line drawn between them subtends at the eye 

 an angle of about one minute, or an angle of about -j'oth part 

 of that subtended by the diameter of the moon. Yet although 

 they exist as close as that, the impression of the two 

 is distinct, and we might have a number of points, each 

 giving a distinct impression. It appears, therefore, that 

 for the purpose of vision it is necessary that stimulations 

 coming from a vast number of independent points, having 

 different bearings from the eye, should somehow or other, 

 give rise to distinct impressions. 



Now if by calculation we trace inwards, to the retina, the 

 course of the axes of two pencils coming respectively from 

 two distant points not far from the centre of the field, it is 

 found that those axes intersect, not exactly in the centre of 

 the eye-ball, but in a point (called the optical centre) a 

 little in front of it, the position of 'which we can calculate ; 

 and the place of either image may be found by joining the 

 external point with the optical centre, and producing the 

 joining line to meet the retina. It is an easy matter now to 

 calculate the distance on the retina of the images of two 

 external points which subtend at the eye a known angle; and 

 it is found that when the external points are so close as only 

 just to be seen as two, the distance of the two images is 

 about the j^^g-oth of a millimetre, just about the distance 

 apart of the cones and rods from one another, lying so closely 

 as I have explained they do. Here, then, it would appear, 

 in this remarkable layer of the retina, we have a provision 

 enabling us to have distinct sensation of a vast number of 

 distinct points in the field of view ; and consequently we have 



