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 newlycoined word 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 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 perspec

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

tive 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 dif ferent 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 86; and the advance of the pupil during the change, 36. In the second instance, the same respective measurements were 8.8, 5·9, 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, 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 these 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 catop tric, 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 trangressions which occur against the rules of punctuation.

REVIEW III.

Hora Subseciva. 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. Pass the threshold of that edifice, and the strife ceases; there you may learn, if the outer world has not taught it, how the science of medicine is not dissociated from the Graces; how the hours gleaned from the labour and trouble of active life may be rendered a means of throwing a halo and a charm over our everyday existence; how harmony and spiritual beauty may be traced in the very weariness of our work, in the restless and unceasing toil which is our lot. It is well for the man who can find such horæ subseciva as have fallen to the share of Dr. John Brown; and the next best thing to filling out a spare hour in the most profitable manner yourself, is to accept the guidance of another who can show you how to do it. Indeed, whether or not you possess the amulet by which you may ward off dull-heartedness or heaviness of spirits, you will equally well bestow the time requisite for the perusal of the Hora Subseciva, for you will find in it how to appreciate the great ones of the past, you will see how much of poetry can be extracted from even the records of a hospital, you will be shown how there is good in all things, if man but knows the process by which it may be extracted. We have no knowledge of the author, but if we may paraphrase l'empire c'est moi, into the "book is the writer," and argue the character of the man from the style of his book, we should pass encomiums upon the qualities of his head and heart, which are altogether foreign to the duties of a critic. We merely allude to this mode of interpretation in order to convey the impressions we have received from the Horge Subsecive. Dr. John Brown presents us with a genial book—a book full of thought, and of that kindly appreciation of the virtues and intellectual characteristics of others, which elevates and warms the heart; while treating earnestly of serious and profound things, he thinks with Horace,

"Dulce est desipere in loco,"

and appears to act upon the principle laid dowu by Autolycus:

"A merry heart goes all the day,

Your sad tires in a mile-a!"

"Let me tell my young doctor friends," he says, in a preface which is as good as any of the set pieces, the overture which conveys the leading ideas to be elaborated in the

opera,

"Let me tell my young doctor friends, that a cheerful face, and step, and neckcloth, and buttonhole, and an occasional hearty and kindly joke, a power of executing and setting a-going a good laugh, are stock in our trade not to be despised. The merry heart does good, like medicine. Your pompous men and your selfish men don't laugh much or care for laughter: it discomposes the fixed grandeur of the one, and has little room in the heart of the other, who is literally self-contained."

In illustration of these views he adds an anecdote, the essential truth of which we can vouch for, because exactly the same thing occurred in our own connection. It is to this effect:

"A comely young wife, the 'cynosure' of her circle, was in bed, apparently dying from swelling and inflammation of the throat, an inaccessible abscess stopping the way; she could

swallow nothing; everything had been tried. Her friends were standing round her bed in misery and helplessness. Try her wi' a compliment,' said her husband, in a not uncomic despair. She had genuine humour as well as he; and as physiologists know, there is a sort of mental tickling which is beyond and above control, being under the reflex system, and instinctive as well as sighing, she laughed with her whole body and soul, and burst the abscess and was well."

Those who with us enter into the humour of this mot de métier, will thank us for another quotation of a similar kind; alluding to Sir Adam Ferguson," whose very face was a breach of solemnity," the author states that he saw him not long before his death at a very advanced age, when he knew himself to be dying.

"There was no levity in his manner or thoughtlessness about his state; he was kind and shrewd as ever; but how he flashed out with utter merriment when he got hold of a joke, or rather when it got hold of him, and shook him, not an inch of his body was free of its power-it possessed him, not he it. The first attack was on showing me a calotype of himself by the late Adamson, in the corner of which he had written with a hand trembling with age and fun, Adam's-sun, fecit.' It came back upon him, and tore him without mercy."

6

Sickness is often used synonymously with sadness, and the atmosphere of the sick chamber is necessarily not the same as the invigorating balmy breeze that passes over the mountain heather; but as we have learnt not to close hermetically the rooms of our patients, and know that all disinfectants and deodorizers yield in power to the eremacaustic effects of constantly renewed common air (formerly common to all but those who required it most), so too we may hope that the time is past when the medical man is characterized by "overwhelming brows," and his presence regarded as a signal for funereal solemnity.

Among the things which Dr. Brown discusses, one of the very few upon which we are disposed to differ from him is the view he promulgates on man-midwifery. He puts it prominently forward, and his arguments have a strong prima facie semblance of justice; but we think that however good in theory, in practice the system advocated by the author would fail. He argues that because in the great majority of births no adventitious aid is necessary, it would suffice for women to assist in all ordinary cases of parturition, the interference of medical men being reserved for those extraordinary cases in which the experience or dexterity of the midwife failed. There can be no doubt that medical men experience a great loss of "time, health, sleep, and temper," in attending upon obstetric cases; but in what way would they acquire that tact and discrimination, that ready aptitude to deal with emergencies which distinguishes the bulk of general practitioners in this country, if they were only called in to assist the midwife once in every two or three hundred cases? Parturition may be a very simple process in itself, but the obstetrician who is not perfectly familiar with its ordinary course will not be likely to appreciate correctly any irregularity that may occur; he may over-estimate or undervalue its importance, and upon the conclusion arrived at may depend the life of the patient. Doubtless in all ordinary cases a good midwife would suffice; but would midwives, however good, inspire the woman in her hour of travail and pain with the confidence and hope that the presence of a medical man carries with it? over, it is to be remembered that the medical man necessarily passes from the house of the poor to the house of the wealthy, and receives remuneration from the one which must indemnify him for gratuitous, or nearly gratuitous attendance upon the other, so that poor and rich commonly enjoy equally careful and conscientious attendance; but this would not be case with midwives; the lady would not consent to employ the midwife known to have come from the pauper's hut, and thus the latter would be debarred from earning such an income as would indemnify her for the anxieties and fatigues of her calling.

More

We have no statistics at our disposal to determine the point absolutely. Neither the Registrar-General nor the leading obstetrical works afford the necessary material, and bearing as it does upon the welfare of humanity and upon the "emancipation" of the female sex, no less than upon the obstetrician, the question well deserves to be ventilated