1S4 ON THE SENSES. 



aperture. This dark ramified figure is nothing else than the net Avork of blood- 

 vessels within the retina of our own eye ; hence it lias been called the "choroid 

 figure." However incredible it may seem to the laity, that we should tlius per- 

 ceive with the eye objects lying on the background of the eye itself, the fact is 

 not the less true nor the less capable of explanation. It is the sliadoics of the 

 blood-vessels traversing the retina which we see by means of the retina. Why 

 they should seem to be outside of the eye — transferred, that is to say, to ex- 

 ternal space, like every image of the retina which excites the brain — we shall 

 hereafter see. If we employ the second method for the production of the phe- 

 nomenon, the explanation will be as follows : the narrow opening of the card 

 presents before the eye a luminous point which sends out rays on every side, 

 and hence a broad pencil of rays through the pupil into the eye. These rays, 

 on account of the close proximity to the eye, are so strongly divergent that 

 both cornea and lens cannot render them convergent by refracting them ; they 

 arrive, therefore, still divergent at the retina, and illuminate it to a considerable 

 extent, exciting, wherever they strike, the sensitive apparatus; hence the bright 

 field of vision. Now appear, as stated, the blood-vessels of the retina like dark 

 shadows on a bright ground, a proof that the sensitive layer of the membrane, 

 on which the light takes efi'ect, must lie behind the blood-vessels, since othei'- 

 wise, these shadows could not be perceived. The vessels themselves run in the 

 layers lying next behind the nervous or internal layer ; consequently, the 

 elements of the retina which enable us to perceive the light and also the shadows 

 as an absence of light, must lie still further back. From the degree of disjjlace- 

 ment which the shadows of- the vessels undergo through a shifting of the ex- 

 ternal source of light, it has been found possible accurately to calculate how far 

 the layer in question must lie behind the vessels, and the distance has in effect 

 been exactly computed which exists between the vessels whose shadows we see 

 and the external or columnar layer of the retina. This is an ingenious and 

 striking proof, which we owe to the same distinguished inquirer who first accu- 

 rately taught us the structure of the retina, together with the nature and con- 

 nection of its elements distributed to different layers. 



If we now proceed a step further in bridging over the chasm between the lu- 

 minous wave and the sensation of light, we come to the intermediate member 

 which has been spoken of in our previous discussions respecting each of tliB 

 other senses — the current, namely, in the excited nerve-fibre, which, having 

 been indirectly set in motion by the wave through the co-operation of the rods 

 and cones in the columnar layer of the retina, flows through the fibrotis extren- 

 ities of the optic nerve in the retina and its remoter channel in the stem of that 

 nerve, to the brain. We have so often inferred from the nature of this move- 

 ment in the nerve its probable identity in all the nerves ; have in so many ways 

 pointed out that this movement in the nerve has nothing at all in common witl 

 the external irritant which occasions it — a fact which, of course, holds gooi 

 with regard to the current of the optic nerve and the undulatory movement of 

 the luminous ether — that we shall spare our readers any unnecessary repetition 

 We may also abstain from anything more than a simple allusion to the whollj 

 unsolved problem of the further destiny of this current, on its arrival at the 

 brain, in the interior apparatus of the excited nerve. We know only that the 

 current of the optic nerve takes effect on this extreme cerebral apparatus, con- 

 sisting of the bulbous bodies heretofore described, in a peculiar manner, induc- 

 ing therein some modification which gives riise in the mind to a sensation of 

 light, just as the corresponding modification in the terminal bulbs of the auditory 

 nerve produces a sensation of sound. Whether any solution of these seemingly 

 impenetrable mysteries is reserved for human ingenuity can only be known to 

 future times. 



Before proceeding to treat of this simple sensation of light in its connection 

 with objective ideas, we must consider the different qualities of the sensation. 



