CHAP, ii.] RIGHT. 519 



are either purely dioptric in function, or are associated with the 

 sensitive visual substances in such a way that the purely nervous 

 structures must be considered as extending no further at least than 

 the inner limbs. We cannot as yet make any definite statement 

 in the one direction or the other. 



In connection with the origin of visual impulses we may 

 perhaps call attention to the remarkable changes which the cells 

 of the retinal pigment epithelium undergo under the influence of 

 light. When an eye has been shut off from all light for some 

 little time the pigment is concentrated in the bodies of the 

 cells, and the remarkable filamentous processes of the cells, with 

 the pigment granules or crystals which they carry, extend a 

 slight distance only between the limbs of the rods and cones 

 (about one-third down the length of the outer limbs of the rods), 

 tinder the influence of light these processes loaded with pigment 

 thrust themselves a much longer way down towards the external 

 limiting membrane ; in consequence a considerable quantity of 

 pigment is found massed between the outer and even the inner 

 limbs of the rods and cones ; indeed the outer limbs of the rods 

 swelling at the same time become jammed as it were between the 

 masses of pigment, causing the epithelial layer to adhere very 

 closely to the layer of rods and cones. 



The retina and optic nerve like other nervous structures de- 

 velope electric currents which may be spoken of as currents of 

 rest and currents of action. They may be shewn by placing one 

 electrode on the retina of a bisected eye, or on the cornea of 

 a whole one, and the other on the optic nerve, or hind part of the 

 eye ball or even on some distant part of the body. They are also 

 manifested by the isolated retina itself. The phenomena appear 

 somewhat complicated by the appearance now of positive, now of 

 negative variations ; but this fact comes out clearly that the in- 

 cidence of light on the irritable retina developes an electric 

 change, the magnitude of which is to a certain extent propor- 

 tionate to the intensity of the light acting as a stimulus. The 

 changes accordingly diminish and cease to appear as the retina 

 gradually loses its irritability after death. We may add that these 

 electric phenomena appear to be quite independent of the con- 

 dition of the visual purple. 



Simple Sensations. 



Relations of the Sensation to the Stimulus. If we put aside 

 for the present all questions of colour, we may say that light, 

 viewed as a stimulus affecting the retina, varies in intensity, that 

 is, in the energy of the luminous vibrations as manifested by their 

 amplitude, and in duration, that is, in the length of time a succes- 

 sion of waves continues to fall upon the retina. The effect of the 



