THE PERCEPTION OF LIGHT. 13 



formation of distinct images of external objects on the retina, 

 lu that formation, as I haA'^e said, the transparent j)ortioii of 

 the eye, the cornea, the aqueons humour, the crystalhne lens 

 and vitreous humour, plays the part of a lens in an optical 

 instrument. I have said the " formation of the images on 

 the retina" ; but the retina is not a mere surface, it has a 

 certain amount of thickness, altliough it is, on the whole, 

 very thin. We may further enquire on what part of the 

 retina, considered at different depths from the place where 

 it first commences, on which of the various layers into 

 which histologists have divided it, is it that we have reason 

 to think that light first acts on the organism of our bodies in 

 such a manner as ultimately to give us the sensation of 

 vision ? 



I have said that the retina, as a whole, though very thin, 

 is not a mere surface. If we go from the centre of the eye- 

 ball 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 essential, apparently, for the growth 

 from its original state, and for the nutrition of the eye-ball, 

 and for the carr}ang on of the process for which it Avas 

 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 Avith 

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

 belie\'ed by anatomists to have relation to the fixing of tiie 

 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 betAveen that and the coats 

 I have spoken of is a very remarkable structure which I 

 shall have to say more about. It is called the bacillary 

 layer. In this part of the retina Ave 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 haA''e been called accordingly rods and cones. The rods, 

 as the name implies, are cylindrical, and the cones are 

 tapering and are somcAvhat of the shape of slender peg tops, 

 the sharp side being turned iuAvards as regards the Avay 

 you look, so that the light, in coming from the outside, first 

 meets the bases of the peg tops, and then goes on toAvards 



