926 FUNCTIONS OF RODS AND CONES. [BOOK in. 



577. Whatever view we adopt, whether photochemical 

 or other, as to the changes which lead to stimulation of the real 

 endings of the retinal nervous mechanism, we cannot at present 

 state anything definite concerning those nerve-endings or the 

 manner of their stimulation. 



Each outer limb of a rod is a cylinder of highly refractive 

 material, closely packed round with the black pigment of the 

 retinal epithelium. When an image of an external object, such 

 as a candle-flame, is formed on the retina, at or near the layer 

 of rods and cones, the rays of light diverge again beyond the 

 focal plane in the form of pencils of rays from each point of the 

 image. Of these some passing between the rods are absorbed 

 by the pigment, while others pass into the outer limbs of the 

 rods ; of these latter some traversing the whole length of the 

 limb, are absorbed by the pigment beyond, while others undergo 

 " total reflection " at the sides, or are absorbed by the pigment 

 after reflection. Hence of all the rays which fall on the layer 

 of rods and cones, a small number only are reflected back into 

 the vitreous humour and so through the pupil ; hence the eye 

 when looked into usually looks black. In the case of the coni- 

 cal outer limbs of the cones the amount of light thus thrown 

 back into the vitreous humour must be still less. We may 

 fairly assume that the light which thus disappears, partly in the 

 actual outer limbs of the rods and cones, partly in their immedi- 

 ate surrounding, sets up changes which, whatever be their exact 

 nature, either are or in some way assist the very beginnings of 

 visual impulses. It also seems probable that these changes, 

 so long as they are confined to the region of the outer limbs, 

 ought not to be considered as nervous in nature, it seems prob- 

 able that they do not take on a nature analogous to that of a 

 nervous impulse, until they have passed the conspicuous break 

 which divides the outer from the inner limbs. But on these 

 matters we have no certain knowledge. 



We may here turn aside for a moment to remark that when 

 an image of a candle-flame is formed on the retina the rays 

 reflected back, as stated above, from the retina through the 

 pupil form a second image in the position of the candle-flame ; 

 hence to see an image of an illuminated retina the observing 

 eye must be placed in the position of the source of illumination. 

 This is the principle of the ophthalmoscope. 



There are many forms of this instrument, but the accom- 

 panying diagram (Fig. 154) will illustrate its essential feat- 

 ures. The rays from the lamp L (or other source of illumina- 

 tion) are reflected by the concave mirror M, M, and brought to 

 a focus at a. The rays diverging from a are, by means of the 

 lens Z, rendered parallel, and thus, through natural dioptric 

 arrangements of the observed eye B, are brought to a focus on 

 the retina at a'. The rays reflected back from the part a' of 



