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II.—Object-glasses and their Definition.

By F. H. WENHAM, Vice-President R.M.S.

DR. MADDOX has obtained photographs of the Podura (Degeeria domestica), by means of a Wales' th, showing a beaded appearance, similar to that observed by myself, and has kindly offered to leave the negatives with some one who will supply prints to those desirous of possessing them, and who have not succeeded in developing such a structure.

The appearance is obtained at the entire sacrifice of the "note of admiration" markings, and that distinctness and sharpness of definition that so delights the eye of the optician, and resembles an irregular ribbing, as if a painting had been made on a panel and the graining comb passed transversely in a slightly undulating course across the ribs, and it is remarkable that where the cross lining has commenced with a larger nodule, this increased size has a tendency to continue right across the scale.

I have recently made some careful examinations of different species of Podura, and cannot confirm Dr. Pigott's statement that the "note of admiration" markings are caused by the crossings of a series of oblique ribs on the upper and under sides of the scale. I find that the best illumination is sunlight: without sufficient reason this is generally repudiated. In cases where the colour correction of the object-glass is imperfect, the chromatic errors are so developed, that accurate observation is difficult; but with a glass correctly achromatic, sunlight is really a valuable aid to investigation.

I have never seen the varieties of Podura so finely shown as by this means. I use no stop under the stage, but throw sunlight on the scale in a direction from the top, towards the root, at an angle of about 30° with the concave mirror. The characteristic markings of the scales are thus splendidly shown by the 5th of a blue colour on a pale-green ground. The highest eye-piece may be used, and the tube drawn out to its utmost length, without sacrifice to distinct vision, thus obtaining a magnifying power of near 11,000 diameters. "The note of admiration" markings are only seen in focus, and then show scarcely the vestige of a beaded structure. By focussing away the markings fuse together lengthwise, and appear like irregular knotted cords-the so-termed "beads." The outline sketch (p. 125) represents an oblique view taken by the camera lucida of the markings on Podura, magnified about 6000 diameters. The margin of the scale (mounted on the covering glass) had curled away so as to bring several of the ribs gradually into profile.

Up to the present time I have been unable to find any of the proper insects in my locality in order to fulfil my intention of torrefying the scales as they lay on the glass, for the purpose of partly

destroying their transparency and rendering them brittle enough to be broken into fragments.

The evidence, as it now stands, that the markings are elevations of the above figure, is as follows:-With the parabolic condenser (which approximates to an opaque illumination) the markings have the same characteristic form, and appear as material bodies of denser structure. Some years ago Mr. Ross showed me a Podura scale, mounted, uncovered, on a black disc, illuminated strictly as an opaque object, by a Leiberkuhn, which he had adapted to a 4th of large aperture. The "note of admiration" markings were finely displayed, and appeared like detached bodies due to surface elevations. This structure was afterwards confirmed by myself by spreading the scales on a highly-polished piece of Daguerreotype plate, and throwing a silver electro-deposit upon them. The detached metal film was then washed with a solution of caustic potash, in order to get rid of the scales, the matrices of which plainly showed the same surface form of markings. The late Richard Beck investigated this test as an opaque uncovered object with a th. The light was thrown on the scale by condensing lenses, and he gave illustrations of the appearances in his usual graphic style and published the following remarks:-"When the markings are at right angles to the direction of the light, they are illuminated on the sides farthest off. When they are in the same direction as the light, with the narrow ends pointing to it, the broad ends appear like brilliant spots; but when the direction is reversed, the light from the points is so slight that the scales appear to have lost their markings altogether. Now, if the object were an opaque substance this result would have been a convincing proof that the markings were depressions; but as we know it to be transparent, it follows that these particular appearances can only be produced by elevations."

In July last year I communicated to this Journal a principle of illumination by laying the objects on a glass surface, from which light was totally reflected, and using under such conditions the ordinary slides upon which objects are mounted. At all points of contact light will be admitted through, giving an analysis of the form of such parts. In the Podura, as the light is thrown on more obliquely at an angle where total reflexion again begins to prevail, the scale appears marked over its surface with bright blue dots,

which arise from the thick end of the "note of admiration" markings being the last to transmit the light, showing that these are real prominences.

With respect to the "Aplanatic Searcher," it would be unfair to Dr. Pigott to discuss its merits, till we have a description before us of its construction and use, so that we can adapt and try it in the same form. I am glad that the optical principle and correction of the objective-glass is now brought prominently forward, and the effects of spherical aberration investigated. I may, however, remark that in the adjusting glasses, as sent out by our best makers, the effects of positive spherical aberration only can be observed, as some range of this is given by the closing of the lenses for the purpose of balancing the negative error produced by glass of different thicknesses. In order to observe the effect of negative spherical aberration on a globule, the index tongue must be taken off the mount, so that the glasses can be separated beyond the range to which they were limited by the maker. This is the neutral point between the two errors, where all aberrant rays are brought to one exact plane, the APLANATIC FOCUS of Mr. Lister.

III.-The Ciliary Muscle and Crystalline Lens in Man.

By J. W. HULKE, F.R.C.S., F.R.S.*

In the course of lectures "On the Minute Anatomy of the Eye which I had the honour of delivering before the College of Surgeons last summer, and which were published in this Journal, I described the histology of the cornea and vitreous humour, of the retina and tunica uvea.

There remain the parts concerned in the accommodation of the eye, and the conjunctiva, sclerotic and optic nerve, an account of which will complete the normal anatomy of the eye-ball.

I propose to devote the present lecture to the apparatus of accommodation.

Let me in the first place endeavour to explain what the term accommodation technically means.

That we cannot see perfectly distinctly at the same instant two objects placed at different distances before the eye is a fact of the truth of which a moment's attention suffices to convince the most unobservant person. The fact is most easily realized when the objects are near, for when they lie at a great distance from the spectator the minor distinctness of one of them is less appreciable, but when they are relatively close to the spectator it is impossible

*Being the first of the Aris and Gale Lectures delivered during the present year at the Royal College of Surgeons.

for him not to become aware of the phenomenon. Thus when I look at the nearer of two trees placed several yards apart nearly in the same line, in a distant field, the minor distinctness of the farther tree is so slight that I may fail to notice it; but when I look at a book through a veil, both being near me and only a few inches apart, I find that when my eye is fixed on the print I see it quite distinctly, while I am scarcely conscious of the presence of the intervening veil; and again, when I look intently at the veil and perceive its texture distinctly, at that same moment the print becomes confused and unrecognizable. What is the explanation of this?

In order to see an object distinctly, an exact image of it must be formed on the bacillary layer of the spectator's retina. Every luminous point in the surface of the object turned towards the spectator must be represented by a corresponding image-point upon his retina. This is effected by the refractive power of the eye-that power its transparent parts possess in common with inanimate transparent bodies, of changing the original direction of the luminous rays which enter it, and of giving these rays a new direction of a kind dependent on their relative densities and their curves.

Luminous pencils coming from remote objects consist of parallel rays, and, having regard to the small opening of the pupil, the pencils which enter the eye from an object 20 or more feet distant may be considered to be composed of parallel rays. Now the refractive power of the eye is such that parallel rays entering it are collected in exact foci upon its retina without the exercise of any vital effort, the eye itself being quite passive. It would occur as well in a dead eye so long as its media remained transparent and while they retained their proper curves.

The luminous pencils which a near object sends to a spectator's eye consist of divergent rays, and the unaided refractive power of the eye which sufficed to unite the parallel rays from a distant object in the bacillary plane of the retina, is insufficient to collect divergent rays in exact foci in this plane. The foci of these rays lie behind the retina which the pencils strike as spots, the sections of cones, called circles of dispersion, and not as points. The result of this is a blurred confused image, and not a clear one, the pro⚫duction of which requires the rays to be brought to exact foci in the bacillary layer.

We are, however, conscious that we possess the power of seeing distinctly near objects as well as distant ones, which proves that the eye has the power to unite divergent as well as parallel rays in exact retinal foci; and this implies the possession of a power of altering its refractive state so as to suit it to the distance of the object we desire to see distinctly, or, in other words, to adapt it to the degree of divergence of the luminous rays entering the eye from the object.

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This adaptation of the refractive state of the eye is technically called its accommodation. It has been at different times attributed to a change of the figure of the eye-ball, to an alteration of the curve of the cornea, and to a shifting of the position of the lens; but more delicate methods of observation than were formerly at the command of the physiologist have shown all these views to be erroneous, and by the direct inspection of the human eye with instruments specially contrived for this investigation, it has been demonstrated that its accommodation for a nearer object is effected by increased convexity of the lens, and this chiefly of its anterior surface, the curve of its posterior surface being altered in a scarcely appreciable degree. With the increased convexity of the lens, its axis is proportionately lengthened, the pupillary region of the iris approaches the cornea, and the peripheral portion of the iris recedes

from it.

There are not any grounds for supposing that this change of figure is wrought by any power inherent in the lens itself, which is plastic, but is not endowed with contractile irritability, and is not dominated by the will being devoid of nerves.

The active factor of accommodation must, therefore, be external to the lens. Now in close relation to the lens there are two muscular organs-the iris and the ciliary muscle. The existence of accommodation in persons from whom the iris is congenitally absent, and its persistence where the iris has been in part or entirely removed, demonstrate its independence in man of this diaphragm. There remains, therefore, only the ciliary muscle as the active agent of accommodation in the human eye. To the ciliary muscle and lens I would invite your attention to-day. I shall take the lens first.

Lens.-The lens of the human adult has a flattened bi-convex figure. The anterior surface is less convex than the posterior, the radius of curvature of the former being nearly twice as great as the radius of the latter surface. The infant's lens is more nearly spherical, which makes the distance between its summit and the cornea smaller than in the adult's eye. This circumstance is not without influence in the causation of the minute white speck on the front of the lens (the central sub-capsular cataract) not unfrequent in persons who have suffered from infantile purulent ophthalmia, even where this has not been complicated with perforation of the cornea. The compression of the cornea by the swollen eye-lids and oedematous conjunctiva, and a slight amount of deep congestion pushing forward the lens, may bring the lens and cornea together, and thus disturb the nutrition of the growing lens at the point of contact, and induce a perverted growth and retrogressive changes in the lens-tissue.

In other mammalia, and in the lower vertebrata, the figure of

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