was that the septa were not straight lines, and not inclined to each other at angles of 120°; but in the posterior surface of the other there were six septa, placed so

unsymmetrically, as in fig. 4, that five of them were in one semicircle, while only one was in the other.

Fig. 4.-Sheep.

When the lenses now described were immersed in distilled water and examined by polarized light, the optical figures which they exhibited were not in the slightest degree disturbed by the want of symmetry in the number and position of their septa.

S

S

It will be seen from the figures, that in numbering the septa I have counted only those from which the groups of fibres, or vortices as they have been called by Leeuwenhoek, take their origin.

If the abnormal structures now described are the result of disease, or of that change in the

8

lens which is produced by age, it is very probable that several of the varieties of structure observed in the human crystalline are abnormal. Reil * thought he saw in a human fœtus, seven months old, siæ septa radiating from each pole, while in adults there were only four. Dr. Thomas Young+ has given a drawing of the human crystalline (fig. 93) in which the fibres occupy ten groups or vortices, separated by ten radial lines extending from the margin to the centre of the lens, each fibre being parallel to their radii, and consequently meeting at an acute angle. In another figure, 95, he represents what he calls ramifications from the margin of the crystalline lens, that is, ramifications of fibres, a structure which is wholly incompatible with his fig. 93, showing the order of the fibres of the human crystalline. No other observer has seen such an arrangement of fibres either in the human crystalline or in any other lens. It is obvious, indeed, that Dr. Young did not possess a correct method of tracing the fibres to their origin, because he makes their arrangement in Fishes similar to their arrangement in Birds, that is, extending from pole to pole like the meridians of a globe, a structure contrary to the results obtained by every observer.

In his 'Icones Oculi Humani, Sæmmering § has given two very minute drawings of the crystalline; and has inferred merely from its mode of splitting after maceration in alcohol, that it divides into four unequal segments, and that in each of these four there are four fissures; and though he afterwards found that by "a careful and continued maceration" in alcohol it separated into fibres, yet he argues that this is no proof that the recent or the living lens has a fibrous structure "like the zeolites!" Had he been acquainted with the existence of teeth in each fibre by which they are held together to give solidity and permanence of form to a soft and semifluid body, he could not have considered, as he does, the crystalline lens to be merely a lenticular humour "like a drop of dissolved gum ||."

In consequence of these contradictory opinions respecting the structure of the human crystalline, I was anxious to study it by means of the optical method which had occurred to me of tracing the fibres to their origin by the diffracted images which they produced.

In some lenses, the age of which I did not know, I found three septa, as in quadrupeds.

In two very large lenses there were on each side four septa, two being placed at each end of a central line, like that which forms the two septa in the Hare and Salmon. This structure is shown in the Phil. Trans. 1836, plate 6. figs. 3, 4, but in the human lens the central line was very much shorter than in these figures. The

* Lentis crystallinæ structura fibrosa. Preside Reil. Defendit Samuel Godofredus Sattig, Silesius. Halæ, 1795, p. 14, 29.

+ Phil. Trans. 1800; or Elements of Nat. Phil. vol. ii. p. 605. plate 12. fig. 93.

Phil. Trans. p. 605. fig. 100.

§ Francofurti ad Mænum, 1804, p. 67. tab. 5. figs. 17, 18.

Instar gummi liquefacti. Icones, &c. p. 68.

structure, however, was so distinct, that I could measure the length of the central line, which was 0.0533, or of an inch.

In four lenses which I examined three days after death, I observed ten septa on each side, but they had no resemblance whatever to the figure given by Dr. Young. The groups of fibres or vortices were like those of quadrupeds, with this difference, that the septa to which they were related were longer and much nearer the margin of the lens. These lenses were taken from male and female subjects about forty years of age.

More recent observers have found equal difficulties in determining the true structure of the human crystalline. Mr. Nunneley, in his valuable paper on the crystalline lens, lately published in the Microscopic Journal, makes the following remarks:

"In the human lens the arrangement of the fibres is the most complicated of any, for while the type is the mammalian tripod, and is best seen in the fœtus, in the adult the planes are more numerous, in consequence of the primary planes immediately branching into secondary, so that a very complicated curvature of fibres exists; the septa upon the two surfaces frequently not being equal, those of the posterior being more numerous than those of the anterior. In the anterior nine septa and radiations are often found, in the posterior surface twelve, which Arnold regards as the more common arrangement in man. This complicity, however, is only in the more superficial layers, for towards the axis the normal mammalian triseptal division is preserved†."

Mr. Nunneley does not inform us whether the statement that the structure varies in the same lens is founded on his own observations or on those of Arnold. Mr. Nunneley's mode of observing consists in "immersing the lens for a few minutes in water at 180° Fahr.," and after allowing it to dry in a warm room, he observes the number of sections into which it splits, and upon the supposition that it splits only in the direction of the septa, he infers the number of the septa from the number of these directions. We cannot place much confidence in results thus obtained. The lens, we think, should be studied in its entire state by following to their origin the converging or parallel fibres, by observing the changes in the diffracted spectra which they produce. By removing in succession the external layers, it will be easy to determine whether or not the structure changes in the layers near the axis.

On the Crystalline Lens of the Cuttle fish.

By Sir DAVID BREWSTER, K.H., F.R.S. L. & E.

The crystalline lens of the Cuttle-fish differs in a remarkable degree from that of all other animals. Cuvier does not seem to have examined its structure. In his Memoir on the Mollusca, he merely gives a drawing of its external form, and mentions that it consists of two parts easily separated, -the anterior part being more convex than the posterior. In the new edition of his 'Lectures on Comparative Anatomy,' published in 1845, the editors, MM. Frederick Cuvier and Laurillard, have repeated almost verbatim the description of the lens given in the original memoir ‡.

Valentin, in Wagner's 'Icones Zootomicæ §', has given a section of the crystalline lens of the Cephalopods, which is repeated in Victor Carus's 'Icones Zootomicæ .' In this drawing the lens is represented as a sphere, the anterior part being much larger than the posterior. In his 'Lectures on Comparative Anatomy and Physiology of the Invertebrate Animals, Professor Owen describes the lens as Cuvier does, as consisting of two distinct parts, the anterior or smaller moiety being the segment of

1

a larger sphere, and the posterior that of a smaller sphere*; and he adds, what is not correct, that "the lens presents the same denticulated fibrous structure, arranged in concentric laminæ, as in the higher animals." Siebold + gives nearly the same account of the crystalline, the lens being spherical, and the anterior of its two halves less convex than the posterior. Mr. Wharton Jones considers the lens as "a sphere divided into two unequal segments, an anterior smaller and a posterior larger ‡."

In studying the crystalline lenses of animals about twenty-five years ago, I had occasion to examine several lenses of the Cuttle-fish, but having found a considerable difference in the structure of different lenses, and not knowing the species from which they were taken, I did not publish my observations in the memoirs which I sent to the Royal Society in 1833 and 1835. As the subject, however, is a very interesting one, and as the attention of anatomists has been lately directed to the structure of the crystalline lens, I have ventured to submit to the Section a short account of my observations.

In the greater number of lenses which I have examined, and which I believe were those of the Sepia Loligo, the lens was not a sphere, as maintained by the greater number of the anatomists to whom I have referred, nor did it consist of two spherical segments of unequal convexity. Its posterior and larger portion was decidedly a paraboloid, and its anterior and smaller portion a spherical meniscus, whose concave surface coincides with the convex surface of the section of the paraboloid. This remarkable form of the lens is represented in fig. 1, where ABC is the axis of the lens, ADCE a section of the whole lens through the axis, DBE the convex surface of the paraboloid, which is spherical, and MNOP (fig. 2) the spherical meniscus, whose

concave surface MON has the same curvature as DBE, the anterior face of the paraboloid.

The following are the dimensions of the various parts of three lenses :

inch. First lens.

AB=0.3433, DE=0.51, AC=0.433.

MN=0.333, BC=OP=0.09.

Second lens.

AB=0.333, DE=0.50.

Third lens.

AB=0.30, DE=0.457.

In some indurated lenses I have found the spherical surface MON slightly convex, and the corresponding surface DBE similarly concave; but I cannot decide whether this structure is abnormal, or that of lenses belonging to different species of the Cuttle-fish. That it was not produced by induration, will appear from the following description of indurated lenses which have been in my possession for twenty-five years, in all of which the faces DBE are convex, and the faces MON concave.

1. AB=0.29, DE=0.433.
2. AB=0.235, DE=0.37.
3. AB=0.19, DE=0.27.

* This description is inconsistent with the drawing (fig. 224), which represents the whole lens as a sphere. In this figure the central part of the lens is represented as not lamellar as its structure.

† Anatomy of the Vertebrata, p. 283. Boston, 1854. Phil. Mag. Jan. 1836. vol. viii. p. 1.

The paraboloidal part of the lens, viz. ADBE, consists of paraboloidal laminæ, the surfaces of which are perfectly smooth, and give no diffracted images by reflexion like the surfaces of the lamine of other lenses. The spherical portion, MNOP, consists of spherical laminæ of the same character. The structure of both portions of the lens is fibrous, the fibres diverging from poles in the axis of the lenses, that is from one pole in each lamina of the two lenses. The fibres, however, must be perfectly flat in order to compose laminæ perfectly smooth; and I have not been able to observe that they are united by teeth as they are in other animals. They must adhere, therefore, to each other by the contact of their surfaces merely, or by some structure not visible in the microscope, and not showing itself by its action upon light.

The thin transparent membranes which cover the anterior convex surface of the paraboloid, and the concave surface of the spherical meniscus, have also a fibrous structure, the fibres diverging from a single pole in the axis of the lenses. The surfaces which these membranes cover are the ends of all the fibres which constitute the lens; and each lamina terminates in a sort of ring or margin which is well defined. This ring is a little larger in diameter than the proper section of the paraboloid, and the consequence of this is that the fibres, or the termination of the lamina, are curved upwards, so that their surfaces are concave near the line DBE.

The two lenses are curiously united. The concave surface MON is less than the convex one, DBE, and there is a notch between Dand M going round the lens. The meniscus is kept in its proper place, in contact with the paraboloid, by a ring abed (fig. 3), in which ab=0.50 of an inch, and cd=0.31.

In the Sepia Eledona the whole lens is nearly spherical, as shown in fig. 4, the Fig 4.-Sepia Eledona.

axis or diameter AB being a little larger than mn, whereas in the Sepia Loligo it is smaller. The lens divides into two parts along maben, abc being a hemisphere about theth of an inch in diameter. The whole annular surface, from the circumference mn to a, c, is covered with two membranes, to which the ciliary processes are attached.

In order to examine the polarizing structure of the lens, I made a section of it by two planes parallel to the axis, so that the plate was about the 15th part of an inch thick. When immersed in oil, it gave, in polarized light, the figure shown in fig. 5, when the axis of the lens po was parallel or perpendicular to the plane of polarization. The tints to the left of v and w were the highest, namely the yellow of the first order. The sections 1, 2, 3, 4 are negative in reference to r, and 5, 6, 7, 8 are also negative in reference to the intersection between 6 and 7. When the polarized light is transmitted along the axis of the eye, a black cross is seen, as in uniasal negative crystals; but the cross opens upon turning round the section, indicating a defect of symmetry in the substance of the lens round the axis. The luminous por tions at 5, 6 are very bright.

When the lenses are quickly dried, the laminæ separate from each other, and the lenses have the appearance of pearls; the resemblance is so great, that when the experiment is well made, it is difficult to distinguish them from real pearls, as will be seen in the accompanying specimens*.

In many of these lenses the laminæ are separated in some parts and not in others, and the consequence of this is, that when we look at the convex surface of the paraboloid by reflected light, we observe a number of luminous and dark rings surrounding the axis of the lens, the luminous rings being produced by the total reflexion of the light from one or more of the separated surfaces. The effect thus produced is very beautiful, as will be seen in the specimens on the table.

The preceding observations were made, as I have already stated, nearly twentyfive years ago, and I have copied them as they stand in the Journal of my Experiments. I observe, however, some discrepancies between the figures and their descriptions, arising, I think, chiefly from not being able to distinguish the lenses of one species from those of another. These discrepancies I hope to be able to reconcile before this communication is published.

On the Use of Amethyst Plates in Experiments on the Polarization of Light. By Sir DAVID BREWSTER, K.H., F.R.S. L. & E.

In order to determine the exact position of the plane of primitive polarization, it was usual to observe when the intensity of the extraordinary image of the analysing prism was a minimum; but as it is difficult to obtain light perfectly homogeneous, the light of this image could not be completely extinguished. In his experiments on the rotatory phenomena of quartz, M. Biot employed a coloured glass, which transmitted only the extreme red rays of the spectrum; but this method, owing to the great loss of light in the polarized pencil, was attended with so many inconveniences, that fifteen or twenty trials were required before he could determine the zero of his instrument. In order to remedy this evil, M. Soleil interposed between the polarizing apparatus and the analysing prism two plates of quartz of equal thickness, the one right-handed and the other left-handed. These plates were united so as to give the same tint when the plane of the principal section of the analysing prism coincided with the plane of primitive polarization. This ingenious apparatus was submitted to the Academy of Sciences on the 23rd of June, 1845, and has been used - since that time by M. Senarmont and others in their experiments on polarization. In the year 1819 I communicated to the Royal Society of Edinburgh the very same ✓ method of placing the principal section of the analysing prism in the plane of primitive polarization; but in place of using two plates of right and left-handed quartz, I used a single plate of amethyst, in which the two kinds of quartz were combined, during the formation of the crystal. This piece of apparatus, which is obviously superior to that of M. Soleil, is thus described in the paper to which I have referred :"The properties of amethyst, which have now been described, render a plate of this mineral a valuable addition to our apparatus for conducting experiments on the polarization of light. If we wish to place the principal section of the analysing prism exactly in the plane of primitive polarization, we have only to interpose a thin plate of amethyst like that shown in the figure, and if the tints of both sets of veins are exactly similar, the analysing prism will have the required position. If the one set of tints is bluer or whiter than the other, or if there is the slightest difference between them, the position of the prism must be altered till that difference is no longer perceptible. If we wish to place a plate of sulphate of lime or any other crystal, so as to have its principal section in the plane of primitive polarization, the interposition of the amethyst plate will give us the same assistance, by indicating that the circular (rotatory) tints are not affected by it; whereas if we wish to place the axis of the sulphate of lime at an angle of 45° to the primitive plane of polarization, the amethyst will point out this position when the opposite circular tints suffer an equal change."

On Professor PETZVAL'S New Combination Lens.

Sir DAVID BREWSTER laid before the meeting a paper, translated by Mr. Paul Pretsch, entitled "Prof. Petzval's New Combination Lens, as an Object-glass for Telescopes," and exhibited a fine telescope for which the lens had been originally constructed. This telescope was constructed for the Imperial General Survey Office at Vienna, for the purpose of making maps, and the lens which it contained has been