14 THE ANNUAL ADDRESS OF THE VICTORIA INSTITUTE. 



the point. About the point of these rods and cones, just 

 close to the choroid coat, is a Layer of pigment 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 extremities 

 reached to the layer of pigment cells formmg a black lining 

 immediatel}^ 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 direction in which you look, so as to reach the tops ot 

 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 munerous. AVhen they are looked on lengthways 

 from the back of the eye Avhen the pigment is lemoved, they 

 form a sort of mosaic. You may imagine the general 

 structure of them by thinking of the head of the common 

 sun-flower in seed. They are aiTanged side by side some- 

 thing like the seeds of the sun-flower ; but they lie so close 

 that the distance between the neighbouring rods or cones, 

 as the case may be is only about (it yaries somewhat fi'om 

 one part of the eye to another) xbVo"^^^ P^^'^ of ^ millimetre 

 or say about -^-j^g-fi^th part of an inch. So numerous are they 

 that a square \\'ith 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 T>Gg 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 laminae 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 of; and although the 

 course of them has not actually been traced the whole 



