120 FUNCTIONS OF RODS AND CONES. [BOOK in. 



state anything definite concerning tjiose 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 conical 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 immediate 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 probable 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 illuminaton. This is 

 the principle of the ophthalmoscope. 



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

 panying diagram (Fig. 150) will illustrate its essential features. 

 The rays from the lamp L (or other source of illumination) 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 I, 

 rendered parallel, and thus, through natural dioptric arrange- 

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

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

 retina thus illuminated, will, as stated above, follow the same 

 path as on entering, and so return to the focus a. Hence the 

 rays reflected from a number of points on the retina, such as 

 those forming the arrow at a', will be brought to a focus in a 



