November 21, 1895] 



NATURE 



67 



outwards, i.e. towards the back of the body, we have first a 

 plexus of very fine nerve-fibres which run along the front of the 

 retina, and ultimately unite in the optic nerve, which runs into 

 the brain. We have also minute blood-vessels, which are essen- 

 tial, apparently, for the growth from its original state, and for 

 the nutrition of the eye-ball, and for the carrying on of the pro- 

 cess for which it was designed, viz. that of enabling us to see. 

 Then we have several layers of pulpy transparent substances 

 which have been called ganglions, nuclei, and molecules, mixed 

 with very fine fibres. Some of these are nerve-fibres, others are 

 believed by anatomists to have relation to the fixing of the 

 various parts of the structure to one another, so that they shall 

 not fall to pieces in the rapid motions of the person using the eye. 



Outside all, at the back surface of the retina, there is what 

 is called the choroid coat ; but between that and the coats I 

 have spoken of is a verj' remarkable structure which I shall have 

 to say more about. It is called the hacillary layer. In this part 

 of the retina we have a vast number of elongated bodies placed 

 closely, side by side. In the human eye, and in the eyes of 

 most animals, they are of two shapes, and have been called 

 accordingly rods and cones. The rods, as the name implies, 

 are cylindrical, and the cones are tapering and are somewhat 

 of the shape of slender peg-tops, the sharp side being turned 

 inwards as regards the way you look, so that the light, in 

 coming from the outside, first meets the bases of the peg-tops, 

 and then goes on towards the point. About the point of these 

 rods and cones, just close to the choroid coat, is a layer of pig- 

 ment cells which absorb the greater part of the light falling upon 

 them. The rods and cones are transparent, and allow the light 

 to pass through them, passing lengthways. I said the extremi- 

 ties reached to the layer of pigment cells forming a black lining 

 immediately inside the choroid coat. That is true of the rods, 

 but the cones do not reach quite so far, i.e. when the eye is in 

 a state of repose, as in darkness ; but under the stimulus of 

 light these pigment cells come down, i.e. forward, in the direc- 

 tion in which you look, so as to reach the tops of the cones as 

 well as of the rods. I have said that these elements (remember, 

 please, that they point radially in the direction in which you are 

 looking, and lie side by side) are exceedingly numerous. When 

 they are looked on lengthways from the back of the eye when 

 the pigment is removed, they form a sort of mosaic. You may 

 imagine the general structure of them by thinking of the head 

 of the common sunflower in seed. They are arranged side by 

 side, something like the seeds of the sunflower ; but they lie so 

 close that the distance between the neighbouring rods or cones, 

 as the case may be, is only about (it varies somewhat from one 

 part of the eye to another) xTnrT^h part of a millimetre, or say 

 about To'sTFth part of an inch. So numerous are they that a square 

 with sides the tenth of an inch would cover nearly half a million 

 of them. 



Now something more about these rods and cones. They are 

 found to be composed of two members or limbs, an inner 

 (nearer the centre of the eye-ball) and an outer. The inner is a 

 transparent-looking body, very much like the other bodies in 

 the neighbourhood. The outer is transparent too ; but it is 

 found to be highly refractive. It is longer in the rods than in 

 the cones. The outer segment of the cones may be repre- 

 sented to the mind's eye by thinking of the metallic point of 

 a peg-top. These outer limbs are in both cases readily detached 

 (when the eye is dissected) from the inner, and they separate 

 after a little into laminte, lying one on the top of the other, 

 perpendicular to the axis of the rod or cone. At the outer end 

 they do not appear to have any continuation, the structure stops. 

 At the inner end (corresponding in the case of the cones with the 

 bulbs of the peg-tops) there come nerve fibres from each of 

 them. These pass through the various layers that I have spoken 

 "f; and although the course of them has not actually been 

 raced the whole way, on account of the difficulty of examination 

 if this pulpy structure, it is pretty certain that they join on to 

 I hose 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 exj^xised to us, the impres- 

 ion is that of a single jxiint of light in the field of view. If 

 !icre be two such points we have the impression of two luminous 

 loints occupying different positions in the field of view. Now 

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

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



NO. 1360. VOL. 53] 



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 -sVth 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 impres- 

 sion. It appears, therefore, that for the purpose of vision it is 

 necessary that stimulations coming from a vast number of inde- 

 pendent 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 xuVsth 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 reason to suppose 

 that the effect of light, whatever it be, on one of these elements 

 (be it cone or be it rod) gives rise to the sensation of a point ; 

 and that the position of that point in the field of view depends 

 upon the position of the element of the bacillary layer which has 

 been affected by the light coming from the point. Moreover 

 in the nerve-fibres which come from the anterior ends of the rods 

 and cones we appear to have a provision for communicating, 

 through the optic nerve, to the brain, the influence, or an indi- 

 cation of the influence which light exerts on one of these 

 elements. 



Now I have mentioned one argument for believing that this 

 remarkable bacillary layer is that in which light, which pre- 

 viously merely passed through the eye as it would through an 

 optical instrument, acts in some manner on the organism so as 

 to give rise to stimulation of the nerves which convey to us 

 the sensation of vision. The argument, so far, is a sort of 

 a priori one, but it has been remarkably confirmed by an 

 experiment of H. Miiller's, made by means of Purkinje's 

 figures. 



When in a room which is not quite dark we look with one 

 eye towards a moderately illuminated wall with uniform surface, 

 and holding a candle to one side of the eye move it up and 

 down, there is seen in the field of view a figure branching like 

 seaweed. This is the shadow of the blood-vessels of the retina. 

 That the candle requires to be moved in order to show the 

 figure, is explained by the consideration that the shadow is not 

 black, but only darker than its neighbourhood, and when the 

 light is steady the exhaustion of the eye for that part of the field 

 which lies beside the shadow tends to equalise the apparent 

 illumination of the parts in and out of shadow ; whereas when 

 the candle is moved the shadow falls on a new place which had 

 been in full light and therefore partially exhausted, and the 

 previous exhaustion and the new partial interception of light 

 falling on that place contribute to make the shadow sensible. 



The existence of a shadow shows already that the percipient 

 layer of the retina must lie behind the blood-vessels. But we 

 may go a step further. By suitable methods of illumination 

 we may cause two spots on the surface of the eye-ball, whose 

 positions can be determined from the circumstances of the experi- 

 ment, to be alternately virtually the sources of the light which 

 casts the shadow, and the places in the field of view of the 

 shadows of the same vessel in the two positions of the illum- 

 inating source can be marked. It is then only a question of 

 similar triangles to determine how far behind the blood-vessels 

 lies the percipient layer, and the distance thus calculated is 

 found to agree, within the limits of errors of observation, with 

 the distance of the bacillary layer as determined by microscopic 

 examination of a dissected eye. 



I have said as you go backward from the centre of the eye- 

 ball, you have, in front of the rest of the retina, a plexus, as it 

 is called, of nerve-fibres lying side by side, something like the 

 threads in a skein of silk, but gradually leading onwards to the 



