tomy, and the fact that this is borne out by entoptical phenomena in placing the vessels at various retinal depths, whereas no membrane, or any continuous surface contiguous to them all, overlays them, there seems to be little or no reason for attaching weight to this phenomenon, if found standing alone in ambiguous significance, in a sense contrary to the concurrent import of those remaining, if they shall be discovered to have such concord. Lastly, the other case imagined, of two shadows from the same radiant, would give birth to two pictures of the vascular plexus, so competing in size that they would breach one another, which does not appear to happen.

Being led, then, per viam exclusionis, to regard the vascular phantom as an immediate projection upon the sentient points, the question remains whether, somehow, the image of the foramen may not be simply a shadow similarly projected. H. Müller's suggestion seems worthy of acceptance, and is to the following effect:

33. We do not see in waving the candle round the eye a uniformly dark circular area, nor a complete circle at all, at any one instant, though we may elicit the whole circle in the course of one revolution of the flame round the optic axis. At any one moment we see a crescent, whose convexity looks to be towards the flame itself, and is really towards the flame's retinal image, this crescent approaching half-moon shape as the image advances towards the foramen centrale, and having a parallax like the vessels have, though of less amount. It is, therefore, the image of the crest of the wall of the central pit of the retina. On the outer side of the crescent there may be remarked a bright beam, which Helmholtz supposes to be reflected from the upper surface of the fovea, but which I rather regard as reflected, as in the case of the vessels, from the brim of the pit. I find that with a = 40°, B=4° for the crescent, when for the same value of the vessel crossing the foramen, which presents the most parallax, gives ß = 8°. From which it results that the brim of the pit is 0.0036, or of an inch, and the vessel 0·0738, or 3 of an inch, from the sentient surface.

a,

From many entoptical trials, H. Müller estimates the interval between the "percipient layer" of the retina and the vessels at 0.17, 0.19 to 0.21, 0.22, 0.25 to 0.29, 0.29 to 0.32 millimetres. In the case of three other observers, 0·19, 0·26, 0.36 millimetres; and these numbers harmonize very well with those I have obtained. Then, from anatomical measurements, he determines that the bacillar layer of the retina in the region of the yellow spot is from 0.2 to 0.3 millimetres behind the vessels, and from the elements of this layer pointing as normals to the retina and collateral reasons, he concludes that they are the percipients.

Chary as we may be in trusting to absolute calculations from such observations as we can make, for the resolution of so principal a question as the primary seat of vision, yet it can hardly be gainsayed that a comparative scrutiny of all the phenomena fairly entitles us to decide that there is a certain anterior placing of the vessels and margin of the fovea with respect to the sentient surface, which appears to point with a high probability to the site assigned.

APPENDIX.

Visual Sentients.

31. Having expended our means of analysis by aid of entoptical, parallax, and allied helps, when we arrive at the seat of the sentients themselves, we must now resort to other expedients in order to get some acquaintance with their differential structure, as affecting the use of sight, and with any troubles of vision that may arise from the working of the machinery of the eyeball. Such knowledge is requisite to protect us against any error that might else creep into entoptical research proper, from a posterior source, as also may lead us directly to a more fundamental idea of the mode in which light stimulates the sentients. I shall proceed as systematically as I can under the altered

circumstances.

If the extremity of the finger or nail be laid upon the sclerotic immediately, or through the lid, even never so lightly, in the nearest attainable regions to the most sensible parts of the retina, we shall elicit a quasi-luminous areola of the contour of the applied surface, though the surface itself seems lit up but in a very meagre degree, or not at all—that is, unless the applied surface be very small, when the brightness of the areola will so encroach upon the included area as to make it difficult to say whether it pales away entirely at the surface itself or not. Thus the resulting image of the surface simulates the shadows of opaque bodies embedded in the retina, with their reflective borders. This marginal effect must, as Young remarks,* be due to the flexure undergone by the retina along that line. He says greater flexure along the contour than upon the applied surface, but I would wish to make the distinction that, though it is obvious that such a flexure as crowds together the internal elements of the retina produces a sensation of light, yet the experiment seems to yield uo evidence that the bare pressure upon its outer surface, or the gathering of its elements closer to one another in the act of depression, has any consequence of a similar kind. Hence, we might infer that it is only the inner surface of the sentient layer which is sensible, or if the cones rather than the rods are sentients, that the rays of light affect them by impinging upon their sides. And if this be the arrangement, that each little sentient surface should be veiled from pressure in a little pit, we might perceive why the most employed portion of the sentient expansion should be walled in at the bottom of an abrupt fovea.

However, the characteristic colour of this and other phenomena from pressure is white, with a tendency towards the less refrangible colours. As if all sorts of lucid vibrations were engendered by mechanical stimulus, with some tendency towards those of slowest waves. The nervous impression in the instance before us is but transitory, but severe compression of the globe produces spectra of diversified orbital structures; and may be made to endure, seemingly along the lines chiefly where flexure such as above has been suffered, quite as long as the spectral image of the noonday sun has ever been known to last.

Philosophical Transactions. 1793.

Not only is the retinal response to objective light injured or annihilated, according to the intensity of these spectra, but we find our vision impaired, or, except for strongly illuminated objects, obliterated by the sole existence of such retinal flexure, as occasions a marginal quasi-light, even though this does not suffice for leaving a discernible spectrum behind it.

35. When we turn in the dark the eyeballs sharply, or even mildly, a couple of white circular rings, brighter at one margin than the other, each enclosing paler area with a central dark spot, flash forth, the diameter subtending an angle of several degrees. They are angularly apart from each other, and from the spectrum of a bright disc planted at the foramen centrale, and enjoy lateral angular play in strict conformity with the received opinion that they originate at the base of the optic nerve. The phenomenon is plainly the result of flexure of the retina where the nerve runs into it, as the eye is pulled round in its socket until it drags upon the nerve. And it is to be noted that it is again where the inner retinal elements are squeezed laterally that the phenomenon is disclosed.

36. In connexion with these facts, it may be convenient to notice an opinion universally adopted by physiologists, from Purkinje to the latest student of the accidents of vision, that the vascular figure, at least for the chief vessels, is rendered visible to us by the motion of the blood in the vessels, and may be thus made more so by pressing upon the contents of the eye through the anterior surface. I have finally convinced myself that all this is but an illusion, though one so deceptive that very much caution is required in trying to unmask it. It is astonishing how little light is necessary to display to us the vascular figure by the rays reflected from the coats of the vessels, especially of the white arteries, as if when the eye receives a very small quantity of light the increase effected where the reflected beams fall is of great moment in producing sensation. And if we carefully darken the eyes the spectral condition of the retina, which is impressed with the images of the vessels (see 32), just like the spectrum of a candle-flame, oscillates a considerable interval before it utterly departs; but if we shut ourselves in a room from which all light is excluded for a halfhour or more, the last remnant of the spectrum vanishes, and then it is in vain to attempt to trace the course of a central retinal vessel. Nor can we do so by pressure upon the ball of the eye. The path of the vessels observed on opening an eye, after being compressed, against the sky, is nothing more than a momentary vision of their shadows by a retina impressed with mechanical spectra. This negative fact again seems to intimate that the retina only shows us quasi-lights when it is creased on its inner side, or at any rate only when squeezed upon hard enough in a radial direction, to yield a similar mechanical effect, in forcing together laterally, by flattening, the internal elements.

37. Yet, even in the darkest place, after any time we discover that false sensations of light forbid us to realize the fact of the total absence of light, because lights reach us in transient whitish or reddish spots, as if from some percussing body, seizing upon any region from the

horizon to the zenith; one or more lucid clouds, if I may so speak, more or less apart, arising in any instant and dissolving in the next. Should we yawn we aggravate the appearances very singularly—that is, we squeeze the retina by forcibly compressing the eye behind the orbital muscle.

Having watched these lucid phantoms against a feebly illuminated ceiling of a room, we soon learn to distinguish them against a cloudless noonday sky, and to ascertain that they are always present.

Now, to the duplicate sclerotic coat of the conjoint globes twelve muscles are attached, eight before and four behind the equator, their inserent spaces so distributed that, when projected into our visual vault, to leave but little of it unrepresented by them. Thus it becomes a puzzling task to extricate from one another quasi-luminous phenomena brought to the general stock by individual orbital muscles. But, after the experience we have had of the instantaneous respondence of the sentients to the slightest touch of the sclerotic, and to a gentle pull of the optic nerve, we should anticipate that the direct strain of these potent instruments upon the tunics would be attended with vivid phosphorescence, as tending to draw them or the retina conically outwards, and thus to bend the inner ends of the sentients together; and a vigilant survey satisfies me that muscular action imparts the greater quantity, if not the whole, of the involuntary phantoms we are contemplating. We see frequent false lucidities after sleep, when the recreated sentients are aroused by the most trifling shock, and these indefatigable muscles are apt to jerk at the ball discordantly; and habitually the balls roll in their sockets to give room to the blood impelled at each pulsation to equally traverse their substance, or for the disposal of the fluids of the conjunctiva. Still, whilst the will does not interfere with these movements, the eyeballs are nearly kept suspended by the muscles, so as to be enabled to preserve their globular form, and are wheeled round smoothly, with so slight resistance to the traction of any muscles, that but exceedingly weak impressions are made upon the sentients. Nevertheless, if the will overrule this mutual consent of the muscles, the result is far otherwise; and if they be made to contract in vehement antagonism to one another, striking events accrue. Then dragging spasmodically at the globe, both before and behind the equator, in spite of the quantity of soft fat which they embrace, these muscles squeeze it against their knotted bellies, and other orbital protuberances, loosening their strain and clonically enforcing it again; not only striving to drag the coats outward at their insertions, but variously flexing the retina, and squeezing the vitreous fluid against it in depriving the eye of its sphericity. The phenomenon in its simpler aspect seems particularly noticeable to me over the region of the broad insertion of the superior oblique muscle, emerging in the visual field within and below the middle point of the field.

As a large artery is included in the trunk of the optic nerve, and two or three others lie near to its outside, some of the quasi-lucidity might proceed from them; but we have no like experience in the

other special nerves, and though fibres of the optic nerve in traversing the retina are flexed when the latter is, yet we see no effect anywhere else than at the exact seat of flexure-that is, none where the flexed fibres terminate; nor does stretching the optic nerve, by turning the eyes, produce lucidities through it. It is possible that some of the minuter spectres might be occasioned by flexure of the retina by the very small choroid vessels. Even the nerves of common sensation are more easily excited at the points of their final distribution than along their course; so that, upon the whole, we seem to have no palpable ground for believing that any of the quasi-lights that assail vision originate posteriorly to the proper sentients of the retina.

Nevertheless, to return to the muscular action, a long suite of illusions originate therefrom. For every surface before the face whose image encounters a quasi-lucid cloud is sullied or expunged. If the eye rests gently upon some point of the wall of the room, patches of the pattern of the paper, the strings by which the pictures are suspended, or whole pictures, are buried in a mist, or entirely extinguished, singly or in groups, from moment to moment. And even if the other eye be brought to the rescue, but partial relief is obtained, for in both retinæ the sentients are very commonly compressed at correspondent regions. In combining two pictures into a solid form by the stereoscope much of this obliteration obtrudes itself.

If we lay a task upon the orbital muscles, as by regarding intently some object placed so near between the two eyes, as to demand an unusual contraction of the internal straight and superior oblique muscles; or if with absorbing effort we gaze towards the hands of another person so waved about before our face that the distressed eyes rather fixedly stare forwards than succeed in following their motions, the above events crowd upon one another, or even absolutely overwhelm the visual faculty. If a landscape is before us, when we strain our eye, and even, with some qualification, both our eyes, as the muscles are thrown into paroxysms of spasm, trees, houses, and fields take their departure here and there, through considerable angles; yet the degree of brightness of the objects tells upon the result-a moon certainly may be quite put out of sight, a sheet of water illuminated by the sun possibly, but the sun itself, in my experience, can never be thus removed. In a room the same phenomena prevail-pens, paper, &c., strewed over the table, flit away as if by enchantment; and even the flame of a candle may be thus extinguished. If we resolutely gaze into the powerful equally diffused light poured into the eye by a half-inch focus lens held very near both to the eye and flame, the rapidly diversifying efficiency of the different sentient patches, under the convulsive warping of the tissues, presents us with a furious whirl and seething of brilliant and grey patches, which is truly astonishing to behold. In this example the lucid cloud is indicated by the darker parts, as less acted upon by objective light. All these visual effects may be obtained just as certainly by squeezing the ball of the eye between the fingers whilst we gaze forwards, as by the strain of the orbital muscles.