we remove it, along with the sclerotic, to which it adheres, we expose a thin fascia closely adherent to the outer surface of the choroid gland and vascular choroid. Removing the vascular choroid and gland, we expose the dense pigment-layer, differing from the membrane of the pigment of mammals only in being more consistent, in consequence of the prolongation of the cells; and under it we meet with a soft grey layer, which exhibits under the microscope the small rods and large bi-geminal cones of Jacob's membrane. In our opinion, the tunica vasculosa is the proper choroid, and must include the tunica Ruyschiana, or capillary network.

That the choroid gland is made up of convoluted blood vessels, as is stated by our author, is true; but the addition of a few lines would have put the reader in possession of the fact, presumed by Albers, but first, through injection and the microscope, demonstrated by Mr. Wharton Jones, that the structure in question is a rete mirabile, or rather the centre of a series of retia mirabilia. The artery which supplies the whole of the gland and vascular choroid, having entered the back of the eye, divides into two branches, which run along the inner or concave edge of the gland, giving off twigs, which by subdivision and anastomosis form a network. The outer or convex edge of the gland is also surrounded by a network of vessels, large enough to be visible to the naked eye. On examining the gland itself with the microscope, it is seen to consist in an innumerable quantity of minute straight vessels, which passing across, establish a connexion between the inner and outer networks. The large vessels of the inner network divide into the minute straight vessels which form the body of the gland, and these minute vessels coalesce in their turn to form the large vessels of the outer network; thus exhibiting the well-known character of a rete mirabile. Many of the vessels, however, derived from the outer network, as they proceed forward in the tunica vasculosa, subdivide into small branches, which again unite, thus forming secondary and more simple retia mirabilia. Lastly, the arteries of the tunica vasculosa terminate in a star-like manner, and the radiating twigs of these stars inosculate together.*

The sixth Chapter, on the Eyes of Extinct Animals, is short and amusing. The seventh, on the Physiology of Vision, occupies seventythree pages, and discusses the questions of optical aberrations, single vision with two eyes, erect vision with an inverted image, complementary colours, ocular spectra, and the like. Seeing the length to which this article has grown, we shall confine our remaining remarks to one or two topics only.

Optical Aberrations-Use of the word Parallax-Monocular and Binocular Parallax.-It is well known that there are three optical aberrations or sources of indistinctness, which must be obviated in the eye, to render vision such as it is-viz., that from sphericity of the dioptric media, that from unequal refrangibility of the differently coloured rays of light, and that from distance of the object viewed. The first is called spherical, and the second chromatic aberration, and by a newly-coined word

* Wharton Jones on the Choroid Gland; Medical Gazette, vol. xxi. pp. 650, 864. London, 1888.

the third is known as distantial aberration. Though distantial is scarcely an English word, yet it is founded on sufficient analogy, and indicates clearly enough the meaning to be expressed. Mr. Nunneley, however, not contented with it, calls the indistinctness arising from distance, the aberration of parallax-a phrase which by many will not be readily apprehended.

By parallax Mr. Nunneley no doubt means the sensible deviation from parallelism of rays emanating from a given point, and falling on a surface of given magnitude, such as that of the lens as exposed through the pupil. If a convex lens is placed at the distance of its principal focus from the recipient surface, it will give distinct images of objects at such a distance that the rays falling from them on the lens are sensibly parallel; but if the object is brought nearer, so that the rays from a point in the object, falling on the lens, become sensibly divergent, there is a deviation from parallelism, or in other words a parallax, which will prevent the convergence of the rays to one point, and consequently exhibit on the recipient surface what is termed a circle of dissipation or aberration; but this can be made to disappear by moving the lens farther from the recipient surface. The application of this to the eye is very obvious; but we see no advantage in substituting the phrase aberration of parallax for distantial aberration. It will require additional explanation before it be generally understood; this, however, Mr, Nunneley has not thought fit to offer. This monocular parallax will require, moreover, to be distinguished from the ocular parallax of Sir David Brewster,* which he regards as the measure of the alleged deviation of the visible from the true direction of objects, and is therefore a totally different matter. Such are the disadvantages of not adhering to the phraseology in common use; by new applications of scientific terms we may make ourselves unintelligible.

From Mr. Wheatstone's paper on the stereoscope, Mr. Nunneley quotes the statement, that when an object is viewed at so great a distance that the optic axes of both eyes are parallel, the perspective projections of it, seen by each eye separately, are similar, and the appearance to the two eyes precisely the same as when the object is seen by one eye only; but that this similarity no longer exists when the object is placed so near the eyes that to view it the optic axes must converge, that under these conditions a different perspective projection of it is seen by each eye, and that these perspectives are more dissimilar as the convergence of the optic axes becomes greater. In this case the angle formed by lines drawn from the centre of each eye to the object constitutes what has been called by the successors of Wheatstone the binocular parallax of the object.

"This angle," says Mr. Nunneley, "supposing the object to be twelve feet distant, and the space between the eyes to be two and a half inches, would be exactly one degree, and at twenty-four feet half a degree, so that it is evident it is only objects near to the eye in which this dissimilarity of images exists, for in the more distant the binocular parallax is too small to produce any effect; in those persons in whom the eyes are widely separated the binocular

* Transactions of the Royal Society of Edinburgh, vol. xv. p. 350.

parallax will be greater, and consequently the appearance of relief proportionally greater, and in those in whom the eyes are near together it will in a corresponding degree be less." (p. 363.)

In illustration of this subject, Mr. Nunneley introduces (Fig. 169) two perspective projections of a cube-the one that seen by the right, and the other that seen by the left eye; but if he will actually look at a cube in the way directed, he will see that the appearances it will present, first to the one eye and then to the other, are the reverse of what he has delineated.

Accommodation to Distance.-Mr. Nunneley discusses at considerable length the subject of accommodation to distance, enumerating various observations which prove the fact; and considering the more prominent hypotheses which have been formed to explain the mode in which it is effected. He omits, however, the proof of the actual occurrence of a change in the eye, according as it is adjusted to different distances, which is afforded by means of the ophthalmoscope, the distinctness of the image of the luminous object on the retina of the observed eye being seen to vary as the eye is adjusted for near or distant vision.

It is strange, too, that our author takes no notice of the highly interesting application of the catoptrical test, as it is termed, to the determination of the method in which the adjustment to distance is actually effected. This subject has been handled in his usual lucid style by Dr. Allen Thomson, in a paper in the Glasgow Medical Journal' for April, 1857, from which, had he had no access to the original memoirs on the subject by the Dutch and German physiologists who first made the application, our author might have derived an accurate notion of the interesting discovery. Even so short an abstract as the following, which we take from Dr. Thomson's paper, would have supplied a striking want in Mr. Nunneley's account of the focal adjustment of the eye.

We select a person under middle age, possessing the perfect power of adjustment, and having placed him in a darkened room in such a position that his head may be kept steady, a small artificial light is held to the side and in front of one of his eyes, at a distance of six inches, and we then examine, at an equal angle from the other side, the images formed by the several reflecting surfaces. These images may be seen with the naked eye, or with a convex lens held in the hand; but Dr. Thomson employs for this purpose a compound microscope, with the erector applied, and a power of twenty diameters. This arrangement allows the eye to be viewed in a steady fixed direction, and the observer to be placed at a convenient distance from the subject of experiment.

The eye not under observation is now to be covered, so as to prevent much shifting of the opposite one, which the person under observation is then alternately to direct to a near and to a distant object, conveniently placed in a direct line before him.

If the pupil be of sufficient size (and if not so it may be moderately dilated by atropine, without destroying the accommodating power),

the three reflected images are seen-viz., one erect, distinct, and large, to the side next the light, proceeding from the cornea; a second, also erect, of a much duller appearance, in which sometimes we scarcely recognise the form of the flame which gives the light, situated near to the middle of the pupil; and a third, much smaller but brighter than the last mentioned, inverted, a virtual image situated between the two former ones, and appearing towards the opposite margin of the pupil from the corneal image. Of these last two images, the former is a reflection from the anterior, the latter from the posterior, surface of the crystalline.

When, by the arrangement of the light, &c., the eye has been brought into such a position that the deep erect image, or that from the anterior surface of the crystalline, is nearer the margin than the middle of the pupil, and pretty close to the inverted image, the eye being at the time adjusted for distant vision, it will be found, that whenever the adjustment is made for vision of a near object, the pupil contracts and advances, and a marked change occurs in the position and appearance of the deep erect image; while the image reflected from the cornea undergoes no change, and the inverted image is only slightly altered. The deep erect image has become suddenly smaller and more distinct, and has shifted its place nearer to the corneal image, or towards the centre of the pupil. The inverted image, or that formed by the posterior surface of the crystalline, has not perceptibly changed in appearance, but has receded very slightly to a greater distance from the corneal image.

The first notice of these interesting facts we owe to Max Langenbeck, of Göttingen, and to Cramer, of Gröningen. The further investigation of them has been carried out by Donders, of Utrecht, and Helmholtz, of Berlin.

The position, size, and distances of the images have been accurately determined by instruments employed by Helmholtz, particularly his ophthalmometer. The following is the result of his calculations, from the observation of the change in the size of the deep erect image in two persons. The measurements are in millimetres, each of which is a little less thanth of an inch.

In one person the radius of curvature of the anterior surface of the lens for distant vision was 11.9 millimetres, for near vision 8.6; and the advance of the pupil during the change, 36. In the second instance, the same respective measurements were 8.8, 59, and 44. The diminution in the radius of curvature of the posterior surface of the lens amounted to 5; but the position of the surface was probably not altered.

As to the mechanism by which the lens is made to change its form, and thus to shorten or lengthen its focal distance, according as near or distant objects are regarded; this is a question on which we shall not enter, leaving our readers to consult the paper whence we have taken the above account of the application of the catoptrical test to the subject of accommodation. Of course the relations and action of the ciliary muscle, and of the other structures at the basis of the iris, 46-xxin.

.4

and surrounding the lens, are directly involved in this question, and still lie open, we think, to new research and discovery.

Mr. Nunneley's style is very unequal, some parts of his work being written with considerable care, while others exhibit marks of haste, and an indifference to elegance, if not even to perspicuity.

Mr. Nunneley's work is illustrated by eight plates, three of which are lithographic, and the other five engraved. Of the lithographic plates, one is coloured. The figures in these plates are well selected, well drawn, and the execution good. One hundred and seventy-nine woodcuts and diagrams are interspersed in the text, most of them taken from works on the same subject, and familiar, like the faces of old acquaintances, to those who have studied the eye and optics. The figures are not at all improved in being copied. No acknowledgment is made of the sources whence they have been taken. Mr. Nunneley makes a sort of apology for this in his preface, as much as to say that the illustrations, and even the ideas, of former authors, have become the common property of the world, and therefore do not require to be referred to their original sources. We cannot subscribe to this sort of doctrine; and, in particular, we like to see such acknowledgments of the pictorial labours of our predecessors, as From Soemmerring, From Müller, After Hannover, Altered from Bowman, and the like. The common property of the world, certainly, these labours are;-to be studied and admired for ages to come. Many of them, such as those of Zinn and the two Soemmerrings, must have cost their authors much time, pains, and money; and when we copy them into our manuals and abridgments, the least we can do is to copy them accurately and neatly, and to acknowledge whence we have borrowed them.

The work before us abounds with orthographical errors; as, catoptic for catoptric, inferiorius for inferius, cribosa for cribrosa, poup for poulp, perinibrachiate for perennibranchiate, and many others. Proper names fare no better; for we have Berkely for Berkeley, Reed for Reid, De Charles for Des Chales, Poterfield for Porterfield, Decemet for Descemet, Horne for Home, Chopet for Chossat, Huck for Hueck, Morgani for Morgagni, &c.

The paper and printing are unexceptionable, but we do think the printers are partly to blame for the numerous transgressions which occur against the rules of punctuation.

REVIEW III.

Hora Subsecive. Locke and Sydenham, with other occasional Papers. By JOHN BROWN, M.D., Fellow and Librarian of the Royal College of Physicians, Edinburgh.-Edinburgh, 1858. pp. 478. THE sacred precincts of the Royal College of Physicians of Edinburgh would appear to shut out the wrangling and contention which make themselves so painfully heard beyond its walls, and jar loudly upon our ears, though in body, and we trust in spirit, far removed from it.