centre of the pupil's image (14), and approaches from the opposite direction, a series of fringes; but if we quickly withdraw a lid which had rested awhile in one place, a luminous band bearing several fringes remains behind, longer than a luminous impression would be extant upon the retina, from a linear accumulation of fluid left by the retreating lid. The convergent pencil modifies these appearances as in other cases. Also, we are aware that a little bar of fluid, as it were a little prism, vertically concave in front from the attractions of the eye and lid, must gather along the edge of each lid, so that in fact the image of this bar in divergent light is projected upon the retina coincidently with the bright space which inflection at the lid would place next its shadow. Thus, by the intervention of the fluid bar, the rays that pass near the lid escape the usual refraction of the cornea, which would bend them towards the pupil's centre, and are either bent less or not at all, or deviate in the opposite direction, whilst light is moreover inflected both at the lid and fluid; besides which, there is a comparatively very weak beam of the pencil reflected from the fluid and lid, which is really thrown across the axis of the pencil towards that margin of the pupil which is on the side of the other lid, from the refracted beam, and may be easily seen as the lid just arrives at the margin of the pupil.

To the refractive powers of the fluid bar are due the upward and downward streams of light that issue from flames when the lower and upper lids respectively advance upon the cornea; to these, with the said inflections, are owing the supernumerary crescents when the lids are winked together nearly parallel to a new moon. If we carry from the lid the back of a knife over, whilst the blade is perpendicular to, the cornea, with a candle so placed that the knife reflects a beam of light into the eye, when we let the back rest on the cornea, the fluid will instantly form the refracting prism described, and corresponding phenomena result.*

The streams are limited pencils, diverging from the eye's surface. By nearly closing the lids we may get such a pencil through the small aperture thus produced. On account of the eyelashes, more than one such orifice may result, or there may be at the same time, if there be a disc of light before us, a pencil from a tear or from reflection at one of the hairs. All these manifest the contents of the deep parts of the eye with its chromatic aberration.

§ III. Iris and Crystalline Lens.

20. Neither the conjunctiva nor the cornea seems to contain any bodies to transmit shadows distinguishable from irregularities on the surface and the fluids thereon, nor can anything be seen in the aqueous humour.

But at a due depth from the tears (8) we encounter the iris, and in a pencil of rays whose focal point is not too near to it, we observe a series of inflective fringes; in light from a brilliant point in advance of the eye, even closing in rings upon the very centre of the pupil's image. With me as with others, this image is not circular. I find it of an oval figure, rather, whose axis is vertical,—and somewhat differing for my two eyes. In my left eye, apparently commencing at the same depth as the iris, and ranging through a depth fully equal to that of the crystalline lens, I meet with many small opaque bodies of irregular outline, fixed in the globe. They are evidently scattered through the substance of the lens. The lens of my right eye is much freer from such objects, but, on the other hand, it is more remarkable for displaying several larger objects which are near its posterior face. One or two of these scem semi-opaque, whilst there are five or six discs of a higher refracting power than the general substance of the lens, and situated more away from its axis. If (speaking in terms of an inch) I introduce (7 and 8) a pair of foci apart as far as the optic centre, so that in the convergent pencils the shadows of a tear ( from the said centre) are retinally apart, and those of the iris (from said centre), these large objects appear in the divergent pencils with shadows showing not more than a retinal separation of their

* In London Medical Gazette, April 21, 1848, I show fin detail how each bright stream is due to a "prism" so to speak, or fragment of a concave lens of fluid" on a lid. Helmholtz, op. cit. s. 146, ascribes the explanation to Meyer, 1853, Poggd. Ann. lxxxix. §. 429. The pale reflected beam, as far as I am aware, has not been described by any one but myself. It was formerly supposed that the bright beam itself was from reflection at the lid.

centres of, which places them as far back from the iris as the posterior vertex of the lens, which should be from the optic centre. Two of these objects have real diameters of and They may be masses of cholesterine or fat like the microscope has encountered in the crystalline, and may have some connexion with the capsule of the lens. The smaller opaque bodies may be earthy particles. They seem to undergo no change. I have been acquainted with two of the refracting specks for eighteen

years.

With a fine divergent pencil a tuft of bright lines, of a straighter character in my right eye than in the left, radiate from about the middle of the pupil, whilst finer ones radiate between them or arise from them. The largest of these lines plainly carry diffractive fringes, and in a convergent pencil form shady dark lines. They are found on either surface of the lens, and (as Dr. T. Young suggested) are clearly caused by the stellate structure of that body, as presented at its two surfaces. Whilst the phenomena mentioned seem to show that the modified substance of which the star is composed must project in its middle line a little above the general face of the lens, so as to constitute a refracting ridge, if we may argue from the strongest (whose ridge may be of an inch in breadth) lines to the weakest ones, for the effects are so delicate, that they vanish away in most of the examples, on the nigh approach of the focus of our exploring lens. From intervening between the anterior face and the retina, the vertex and limbs of the posterior star are the more visible, the limbs of the other seeming to peep between the hinder ones. The image of the predominant vertex usually deviates from the middle of the pupil In my case, for a lucid point at my reading distance, it appears as if lying at about of the vertical axis of the pupil from its apparent lower edge, and in my left eye slightly towards the apparent left edge. Besides, there is a pervading dotted faint cloudiness of the lens or capsule, which it would hardly be possible to analyse.

=

If we introduce a pair of foci to the posterior surface of the crystalline lens, and in equation (b) from 2 in the sense of 4, make F' G', and E' r' = of the optic axis, we find that an object falling in the pair of convergent pencils at the anterior surface of the lens, would have c' D', the separation of its retinal shadows, equal to 5 A B. If we now suppose the retina to retreat from the lens by of the optic axis, or F' G'}}, C'D' = 1336 A B nearly, giving a separation of the two shadows 4th further asunder. By comparing the separation of the shadows with that between two of the globules that are observed to be attached to the hyaloid by short threads, we might readily detect such a deviation as the one imagined, did it take place. It is deduced by Dr. Young that the accommodation of the eye from vision for infinite distance to that of nearest distinct vision, through an alteration in the length of the optic axis, would require the axis to be lengthened by one seventh of itself. It thus appears that did all this extension occur between the lens and the retina, which the properties of the latter forbid us to accept, we could entoptically detect such an alteration. But if the axis lengthen much in its anterior portion, so that objects in or upon the eye move much, where we can easily bring the foci into their vicinity, it will be found by substituting the proper numbers in equations (a) or (b), that very considerable deviations in the separations of the pair of shadows result. In a word, any change in distance between a tear and the iris, between the latter and a speck in the lens, between any two of these specks themselves, at all approaching one-seventh of the optic axis, could not fail to be observed by the methods adduced. Hence the fact of our not being able to readily ascertain (I am not sure that we may not detect some movement between the tears and iris or bodies in the lens) that the pair of shadows after the accommodation, do vary their apparent distance from each other, in a negative fashion, supports the opinion of Young, Helmholtz, &c., that focal adaptation takes place by a change in the form of the lens, as that supposition is the only one which requires a very minute (3th of an inch in the lens, says Young) alteration in length along the optic axis.

23. Young, and all subsequent observers, notice a want of symmetry in ocular refrac tions, so that the focal adaptation which would bring the rays from a lucid point which fall in the vertical plane most accurately to a point, is not the same as that which brings

* Phil. Trans., 1801, p. 53.

those of a transverse plane most so. talline lens. 24. The inflective fringes of the edge of the iris involved in ocular chromatic dispersion, and the stellate figure of the crystalline lens, wherever any portions happen to be parallel to a linear bright space, tend to arrange themselves along it, so as to form supplementary images of it, to do this, through the boundary lines, to letters in a book. And wherever a series of alternations of dark and white lines occur of a certain breadth, especially if the lines be curved, to cause such effects to cross one another. Thus, whenever we stare at such a series until the pupil dilates and the eye loses its focal adjustment for their distance from it, a very singular exhibition of broken, or intercurrent white and coloured lines results.*

Young ascribed this to the influence of the crys

§ IV. The Vitreous Body.

25. Perhaps it would help my readers to a readier apprehension of the chief points to be kept before the mind in investigating the nature of the vitreous body, if I place before them, as preliminary to entering upon details, something like a definition descriptive of my views of the visible texture floating within it, by which entoptical researches are guided, and the structural inferences to which I have been led after a diligent study of its movements.

From the walls of the cavity behind the crystalline lens, as far as it is lined by hyaloid membrane, there springs into view a lax network of beaded fibre, which is the frame of an invisible membrane that divides the peripheric portion of the vitreous into a certain number of little chambers, separating them from each other, and from a larger middle one. These compartments are filled with fluids of graduated density, in such order, that the densest lies next the capsule of the lens, and the rarest next the retina, so that the vitreous body is a compound optical instrument, whose anterior constituents excel the posterior in refractive power.

26. Before we enter upon the exploration of our visual organs for the verification of this statement, let us consider for a moment what forces are likely to act upon a reticulation thus attached in a cavity filled with fluid, so as to occasion its parts to migrate. First, it must be remembered that when the globe rotates, the fluids within will relatively rotate. No matter about what axis the rotation ensues, whether about one through the globe's centre by turning in its orbit, or about any other by the movement of the head, or of our whole person, the vitreous fluid will strive to abide absolutely at rest; or, translation of the orbit apart, rotate within its cavity, through the same angle. But in consequence of the obstructions from the web, if not from some friction between the fluid and the parietes of the cavity, the latter must ultimately concur in the rotation. And as to the objects visible in the fluid, their connexions must exercise control over their movements. Thus we have a modified result, or upon the whole, the objects in the vitreous will travel in the direction of the rotation, but will start from their places at later respective instants than the beginning of that act. Then go equally with the rate of rotation. And when the ocular rotation is arrested, they will continue theirs awhile longer, through the inertia of the now rotating fluid, until the fluid ceases to move, or until they reach the ends of their tethers; when, after a momentary pause, they tend to regain their original sites by retraction, or the web's elasticity. Independently of these connexions, rotation of the fluid in its vessel would cause the objects nearest the wall to journey through the greatest linear spaces, and the others gradually through less, unless at the centre of the entire chamber they would remain at rest.

As I gather from Helmholtz's reproduction (Encyk, cit., s. 152), Listing (Beit. z. phys. Optik. 1845) pictures the entoptical characteristics of the crystalline lens just as I have done. But the larger bodies which he describes corresponding to those that I have above given, he connects with the anterior portion of the capsule, and apparently also the smaller ones which I regard as dispersed throughout the lens. He speaks of the stellate figure as owing to "dark radial lines." He also sketches a sort of "irregular star" of bright stripes (Streifen) issuing from near the middle of the pupil, which he conjectures to have been occasioned by the separation of the capsule from the cornea in the fœtal state. These nice distinctions with the entoptical method he used cannot, I am persuaded, be safely made, and I am rather disposed to suspect that even the irregular star was an aspect of the true stellate figure on one of the faces of the lens. Helmholtz seems to imply that the manifold images of lines, &c., through the stellate figure was explained by Gut (Uber Diplopia monophthalmica. Dissert. Zürich, 1854). I do not know if Gut, or any other observer, had noticed like effects from the iris.

Though the principles here sketched are found on trial to be generally agreeable with the movements of the bodies visible in the vitreous, yet we discover that their operation is disturbed by the influence of a force causing movements solely in the vertical direction, at the instant, whatever point of the eye's circumference happens to be then uppermost; and which must therefore proceed from a difference in specific gravity of some of the contents of the vessel.

If the axis of the eye be horizontal, whether we stand erect or lie on either side, or let the head depend, and we now look to a higher point, the objects in the back part of the vitreous will actually (6) rise, as far as their connexions permit, exceeding the angle of rotation far beyond what happens in horizontal rotations, through a considerably greater angle than the axis has turned through; whilst those in the front part of the vitreous will actually descend through angles equal to those risen through by the former objects, which are severally at corresponding distances from the wall of the cavity. But if from a horizontal direction of rest we turn the regard to some lower point, both the anterior and posterior objects so accompany the movement as not to rotate at all in the eye. And similar effects respectively take place either in looking from a lower to a higher point, or vice versa, no matter through how great an angle.

Hence the posterior objects are, relatively, so strongly buoyed up, and the anterior ones pulled down by a difference in specific gravity in some of the contents of the en tire chamber, as to more than counterbalance the effects of friction in the rotatory act. Nevertheless, with the optic axis vertical, whether we look upwards or downwards, neither the objects nearest the foramen centrale retinæ, nor those nearest the lens, approach appreciably, or depart from the wall of the vessel respectively nearest them. And it is as much as ever we can detect a very slight vertical movement in objects rather further removed from the wall than either of the examples mentioned. As, therefore, the difference in specific gravity which we have found to exist does not manifest itself decidedly under circumstances when it must have a great tendency to do so, the objects we regard must, immediately or mediately, in some fashion be tied to the parietes of the cavity.

When we wish a deliberate inspection of an intra-vitreous body, we had better look downwards, because this is a direction agreeable to the ocular muscles, and the body will remain steady whilst we survey it.

In setting about the scrutiny in question, the first glance conveys to the mind the impression that the floating bodies we behold are a confused mixture of globules and fragmentary filaments. The most cursory methodical exploration apprises us that all these behind the crystalline lens, and by 3 and 4 we resolve that whatever be the breadths of the shadows, under various conditions, all, from front to back of the vitreous, whether filaments or globules, have a general agreement in breadth, so that both in a divergent and convergent pencil increased magnitude in the image shows a less distance from the focal point we employ; though in any given region globules or filaments, side by side, at the same focal or retinal distance may differ appreciably in size.

If, to get more accurate information (speaking in terms of an inch), I place a pair of foci apart at the optice centre (7 and 20), the formost fibre in the vitreous, casts in the divergent pencils shadows whose middle lines are retinally apart, and T broad. It is thus from said centre, or from the lens, and has a real diameter 10. The anterior current of fibres seems to extend to fully from the lens, and the breadths of the main ones to be about that of the example chosen, or a little less. With foci apart, one of the most advanced fibres of the posterior current, retinally shows shadows apart and broad; another, behind this one, shadows apart and broad. These are therefore and from the retina, and have real breadths of and I may see a fibre lying over the punctum aureum retina within of it, having a parallax little more than the central vessels of the retina, which are about To from the sentient surface, from which we are, in fact, measuring, on movements "of a pencil parallel to the retina (10). We must guard ourselves against taking for globules little vascular dots or specks in the hyaloid or tissues between it and the sentients which are brought into view by such a movement as will be explained in the next section.

The image of the globule in a divergent pencil is a bright circular area surrounded by diffractive fringes; in a convergent one, a dark circular area, within a fringed bright areola. The globule, therefore, is transparent, and of greater refractive power than the fluid in which it floats; but it cannot be of much greater, for were it so globules residing near the retina would evince shady centres when we receive into the eye rays from a lucid point placed as far, or nearly as far, away from it as its anterior focus (14); for they should then be able to bring the rays that pass through them to a focus nearer to themselves than to the retinal seat of their image, and thus to project them away from the image's middle portion. However, in such a case, we may observe globules near the lens with obscure middles; and what has been said of the globule may be considered to apply to the filaments.

The objects near the crystalline lens are easy to see, and therefore could not be attached to its capsule without its being noticed. Whereas, on the contrary, every bead or filament thereabouts sweeps across the pupil, however broad this be, without any check.

If we now single out, with a divergent pencil, one of the smallest images, or a bead nearest the sentient points of the retina, and move the eye about gently, we can note that it accompanies the movement; whilst others near it, with broader shadows, pass over it and swim about. If the eye rotate through a very small angle, it does the same, though the others referred to remain at rest, not yet having been set in motion. And when the eyeball rotates sufficiently to cause the mass of globules, less near the retina, to appear to move, they will proceed even after the eye has ceased to move, and will regain their places by apparent retraction; whereas, the object we are especially watching moves, if not exactly, all but exactly, with the eye, and stops with the eye. If we reflect that the smallest instance of images ever lessening for more posterior objects scarcely expresses the lucid and shady contrasts which reveal it, we can well understand that a like object behind the one that yields such an image would not be discernible at all. So that if all the forthcoming observations shall be found to harmonize with the notion, we may conjecture that a chain of one or more invisible beads is the mode of connexion with the parietal membrane, ending in the "decumbent nuclei" that histologists find thereon. Especially if there is reason for believing that none of these threads spring from that portion of the membrane that covers the punctum aureum retina.

If we now turn in succession to objects at different depths in the humour, we observe that their several excursions from positions of rest, consequent upon ocular movement, are in accordance with what we should expect from objects tied together as imagined, under the dynamical influences actually in play. In so far that, though we might never be able to divine how the seemingly solitary globules are retained in their relative places, we might be sure that they are parts of a system.

we

But if we repeat the examinations of these objects in various postures of the head; if rotate the eye in all the positions it can take, gently, quickly, and strenuously, seizing every advantage for getting as extensive and diversified a sight as possible of the contents of the vitreous, then we shall readily come to see that a host of fibres start from the hyaloid, from place to place, as far over it as the super-imposed retina has a moderate sensibility, and congregate into fibres that stretch out into the fluid. Thus, reversing the description for a given instance, by the gradual tapering of the image through variation in retinal distance, in a divergent pencil, a fibre coming from the depths of the vitreous when the eye is prone, to attach itself above, wears the aspect of a trunk of a tree throwing out branches and twigs, holding by the ultimate subdivisions to the retina, so that in a vehement rotation of the eye the tree seems to fly along space, revolving on its twig-tips. And further, it requires but the bestowal of a little pains to make ourselves conversant with the skeleton of a fibrous network stretching all along the walls of the vessel.

Nor can we have omitted to notice that many of the filaments present to us uninterrupted ranks of beads of much length. If closely observant, and our pencils of the best, we perceive that every filament in a plane parallel to the back of the eye, or crossing the axis of either kind of pencil at right angles to it-that is, at least approximately intersecting the rays of the pencil in this style, is resolved into a thread of beads. If the