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. himself acknowledges (p. 64), but does not seem to be aware that it might have been obviated by following a better method.
Vagueness of Popular Science.—Mr. Nunneley announces (p. 35) that h " purposes "briefly to illustrate" in chapter third "the construction and action of the more common optical instruments." He adds: "No mathematical formula, nor any abstruse calculations will be introduced." To such popular explanations there can be no objection; only they must be comprehensive and correct. This is not the case with Mr. Nunneley's statement (p. 44), that "all images formed by concave mirrors are inverted, and generally positive." Any one who looks at his face in a shallow concave mirror, knows that ho sees his image not inverted, but erect. Such a phrase on such a subject as "generally positive," is altogether inadmissible. It must be stated when the image is positive or real, and when it is virtual or imaginary, and the meaning of such terms as positive and virtual must be explained before they be used, which Mr. Nunneley has not done. With respect to concave mirrors, a few additional words would have saved a glaring error, and avoided all vagueness. Images formed by a concave mirror appear before it, are positive, diminished, and inverted, except when the object is placed nearer to the mirror than its principal focus, in which case the image is virtual, magnified, erect, and appears behind the reflecting surface.
Another example of a similar sort occurs in the following statement:—" In consequence of this bending of the rays as they pass through a convex lens, it is an universal rule that an image so formed is in an inverted position relatively to the object." (p. 58.) Any one who looks through a reading-glass or a pair of convex spectacles, knows that the letters of the book he reads do not appear to him inverted. Hence it is plain that Mr. Nunneley's rule is not universal, but requires to be corrected. It is only when the rays, bent as they pass through the convex lens, converge to actual foci, that the image is inverted. If they diverge from a virtual focus, the object and the image subtending equal angles at the centre of the lens, as is the case in the instance of the reading-glass, convex spectacles, or a simple microscope, the image is erect.
Use of Terms.—" From this property of light [refrangibility] arise," says Mr. Nunneley, "two most important laws, which are of so general application (ceteris paribus) as to constitute axioms." (p. 47.) Here the term axioms is misapplied. An axiom is a self-evident theorem, the truth of which the mind admits as soon as the words in which it is expressed are understood; but the facts or laws referred to by Mr. Nunneley require to be demonstrated by experiment, and are therefore of a totally different nature from axioms.
To some of our remarks, Mr. Nunneley may perhaps be disposed to say, "Oh, this treatise is not meant to be strictly and scientifically accurate. Why, we would ask, s should it not? Would it not be better to take a little additional pains, and make it accurate so far as it went, than to leave the student in a maze of unexplained terms or an actual flood of errors? We object particularly to the using of technical terms, the meaning of which is not generally known, and which is left unexplained. For example, Mr. Nunneley states correctly, that" it was formerly supposed that the angles of incidence and refraction always bear the same proportion to each other. This, however, is not the case, but it has been ascertained that the sines of these angles always bear an exact proportion to each other." (p. 50.) He gives no definition of what a sine is—no explanation of the fact that the sines of angles have not the same proportion to one another as the angles themselves. "Principal focus," "conjugate foci," and various other terms, are introduced by Mr. Nunneley without any previous explanation.
"It is unnecessary," says Mr. Nunneley, "to enter upon the rules for finding the precise degree of refraction which each of these forms of lens produces upon the different rays of light." (p. 53.) We should say it is necessary; only if it is meant, however, that the subject should be understood. No one can be said to understand any subject in physics unless he knows the rules belonging to that subject, and knows them mathematically. He may learn, like a parrot, to repeat a number of words or sentences about the matter in hand, such as the law of the sines or the general effects of the different forms of refracting media, but he does not know it as a matter of science.
Diameters of the Eye—Place of the Punctum Cacum.—In his fourth chapter, after
a description of its appendages, the author proceeds to the globe of the eye, remarking that "the human eye is described by some anatomists as being a true sphere, but by the greater number as being a spheroid, the diameter of which, from before to behind, is somewhat greater than in any other direction." (p. 127.)
"I am much inclined to think," he goes on to say, "whatever departure there is from a spherical figure is in the contrary direotion to that which it has been so confidently and commonly stated to be, and that the opinion of the antero-posterior diameter being larger than the transverse has rather been formed from looking at a section where the projection of the smaller curve of the cornea, set in, as it were, to the somewhat flattened termmation of the sclerotic, gives rise to the appearance of greater projection, than to the actual measurement of the two diameters." (p. 128.)
He then enumerates many mammals, birds, reptiles, and fishes, in which the anteroposterior diameter is shorter than the transverse, without noticing, however, the bat, in which the reverse is strikingly the case,* so that while the habits of this mammal resemble those of a nocturnal bird of prey, so does the form of its eyes. He has taken much pains in measuring a great number of eyes of men, women, children, and inferior animals. The tables of measurements which he subjoins are of importance, as showing also the form and size of the cornea, and in pointing out the place where the optic nerve enters the sclerotic, and its distance from the axis of the eye.
Mr. Nunneley shows that in man, and a great number of other animals, not only does the optic nerve enter to the inner side of the axis,' as has generally been noticed by anatomists, but that its entrance is, both in the lower animals and in man, below the level of the axis. That this is the case in the lower animals has, we think, been generally observed; and all the ophthalmoscopists, in their representations of the interior of the human eye, have shown the papilla conica as not only to the nasal side of the macula lutea, but below its level. That the situation of the entrance of the optic nerve does not appear very strikingly below the level of the axis, in the human eye removed from the orbit, is shown by the fact that Mariotte had set it down as entering higher than the axis. He says:—
"J'avois d'aillenrs souvent observe* par l'anatomie tant des hommes qne des animanx, que jamais le nerf optique ne repond justement au milieu du fonds de 1'oeil, e'est a dire, & l'endroit oil se fait la peinture des objets qn'on regarde directement; et 'que dans l'homme il est nn peu plus haut et a cote, tirant vers le nez."t
Accordingly he placed the piece of paper the image of which he meant should fall on the punctum caecum, below the level of his eye. Dr. D. Griffin, in his experiments on the punctum caecum, found a tendency in the eye, in moving outwards, to move also a little upwards; from the circumstance that the image, or candle, was more perfectly hidden then, than when the axis of the eye was directed to a point in the same horizontal line with it. "This," says he, "seemed to indicate that the punctum caecum was situated a little higher than the extremity of the visual axis."J Now, it really indicated quite the reverse; the eye tended to move upwards, that the image might fall below the axis, on the end of the optic nerve. Instead of 1° 11' being the probable elevation, as Dr. Griffin states it to be, of the centre of the optic nerves above the plane passing through the visual axis of both eyes, there can be no doubt that this is the depression of the same point below that plane. Accordingly Mr. Nunneley, on measuring in the human eye the distance from the centre of the optic nerve to the upper margin of the cornea, found it greater than the distance from the same point to the lower margin, in the proportion of 1*9 inch to VI inch, and in some cases still more. His measurements show that a difference in this respect exists in some individuals between the two eyes; and, what is still more remarkable, that in certain rare instances some of the
* "Axis bulbl dlatnetro horizontal! longior; cornea lnslgntter convex*, homisphaerium anterlus fere totum occupat, ita nt scleroticam magnitudlnesfere sequet."—D. W. Soemmerring, De Oculorum Hominls Anlmaliumque Sectlone Horiiontale, p. 26. Goettinge, 1318.
t Nonrelle Dceonverte touchant la Vno: OliuvTes do Mariotte, p. 494. Lelde, 1717.
t Contributions to the Physiology of Vision; Medical Times and Gazette, vol. zxli. p. 283. London, 1838.
measurements from the optic nerve to the cornea are the reverse of the normal. Thus, in one case, the measurement from the optic nerve to the inner margin of the cornea was l}£ths inch, while from the same point to the outer margin was only lT\ths inch. This was the case in the right eye only, the measurements of the left being the usual ones. The person to whom these eyes belonged had been noticed when a patient in the infirmary, to have a peculiar manner of holding a book or newspaper to one side, and very near his face. The probability is, he read only with the left eye, the punctum caecum of his right being in, or nearly in, the axis of vision. In the examination of cases of congenital imperfection of one eye, the possibility of suchf an occurrence should be kept in mind. In two foetuses, the one at full time and the other at five months, Mr. Nunneley found the measurement from the optic nerve over the eye to the cornea less than that under the eye to the cornea. This was the case in both eyes of each foetus.
Proportions of Sclerotic and Cornea.—In the two following statements, Mr. Nunneley has fallen into error. "The sclerotic forms about five-sixths of the external wall of the globe." (p. 150.) "The cornea forms about one-sixth part of exterior tunic of the eye." (p. 154.) We know it is stated, and perhaps it is stated correctly, that in a horizontal section of the eyeball, five-sixths of the circumference of the section are formed by the sclerotic, and the remaining sixth by the cornea. Now, of all such sections, the areae of the spherical surfaces are to one another as the lengths of the included portions of the diameter. The proportion, then, of the surface or external wall of the globe which the sclerotic bears to the cornea, will be as one hundred and eighty-six to fourteen nearly; or, omitting fractions, as thirteen to one. The cornea forms, not about one-sixth, as Mr. Nunneley states it to do, but about one-thirteenth part of the exterior tunic of the eye.
Corneal Tubes.—If a puncture, as was discovered by Mr. Bowman, be made near the edge of the cornea of an ox, or other large mammal, and the month of a mercurial injecting tube be inserted, the metal runs along in parallel.and delicate lines for a short distance, to diverge at different angles into other similar channels, crossing the former at different depths in the cornea. Whether this system of canals is concerned in endowing the cornea with its necessary transparency, or in promoting the nutrition of its non-vascular tissues, has not been determined ; but in exhibiting such a structure, in the manner described, we have never found any difficulty. Mr. Nunneley, however, has failed to observe any such special arrangement of tubes, and does not believe (p. 160) in its existence.
Anterior Elastic Lamina.—This layer of the cornea, first described by Mr. Bowman, and regarded by him as the basement membrane of the conjunctival^pithelium of the cornea, and the counterpart of the membrane of Descemet, Mr. Nunneley has looked for (p. 161) most carefully, without having satisfied himself of its existence. Certainly, the so-called anterior elastic lamina cannot be viewed as the counterpart of the membrane of Descemet. There is on the posterior surface of the proper substance of the cornea a condensation of tissue, similar to that on the anterior; but neither the one nor the other ought to be looked on as a distinct lamina. The smooth dense surface exposed by removing the epithelium anteriorly, or the membrane of Descemet posteriorly, can no more be regarded as distinct from the proper substance of the cornea, than can the outer and inner tables of the skull be, properly speaking, regarded as distinct from the intervening diploe.
Pigment.—In describing the pigment of the choroid and iris, Mr. Nunneley entirely confounds the cells with the granules. He acknowledges that, under the microscope, hexagonally-shaped bodies are seen forming the membrane of the pigment; while on the outer surface of the choroid, and incorporated with its substance, bodies approaching to a stellate shape are discovered; but that these are true cells he does not admit (p. 170). On teazing out the stellate bodies, ho finds, of course, a great number of granules, but he insists on calling them cells. He describes (p. 173) the well-known molecular motion of the granules, bodies which are seen in myriads when the pigment is broken up under the microscope, but he insists on their being true cells filled with fluid.
The generally received doctrine is, that pigment, both in vegetables and animals, is contained in the interior of cells, and that it may exist either in a fluid-or in a granular state; that the granular state, while extremely rare in vegetables, is common in animals j that the cells containing the granular pigment, have walls of structureless membrane; that they vary both in size and figure, the most common form being hexagonal, with a diameter of about ToVoth inch, while in other instances they are larger and of a different form, being elongated and branched, so as to present more or less of a fusiform, bifurcated, or even stellate appearance; that the colour of the cells depends on the quantity of grannies which they contain, bodies measuring on an average j^i^uth of an inch in diameter; that in newly developed cells, such as those of the membrane of the pigment in the foetus, their nucleus may be seen as a white spot in their centre, while in older specimens it is seldom plainly seen, having either disappeared or being hid from view by the great abundance of the granules; and that one of the most distinct examples of pigment-cells is found in the hexagonal bodies of the choroid epithelium, and of granules in the contents of these bodies.
It shows a degree of perverseness on the part of Mr. Nunneley, to insist on calling the granules cells, and to deny the true character of the hexagonal and stellate bodies of the pigment. He states his belief, that the granules are identical with the pigment-cells of the rete mucosnm, and the colouring matter in melanosis; but every one knows that such is not the case, and that the rete mucosum, or last-deposited layer of cuticle, owes its colour to the presence of cells, containing granules of pigment, and that melanosis also consists principally of cells filled with black granules.
Muscce volitantes.—Our author follows up his description of the choroid pigment by stating his belief (p. 174) that its arrangement affords a satisfactory explanation of what are termed mmcoe volitantes; to the appearance of which, as seen by the eye of the patient, he thinks a portion of teazed-out choroid bears a perfect resemblance, and therefore that the cause of this affection resides in the nodulated masses of pigment, with their connecting and stellate fibres. Attributing the stellate arrangement to the aggregation and attachment of the pigment to the vessels of the choroid, he ascribes muscae which change their form, to congestion or contraction of the capillaries, or to the deposition or absorption of fibrin, while those that are constant he refers to some organized deposit or permanent capillary alteration.
With respect to fixed muscce, without pretending to decide that they never arise from morbid changes in the choroid, we believe they generally depend on lesions or deposits in the substance of the retina; but so far as the more common kind of muscce, namely, the floating ones, is concerned, a satisfactory answer to our author's hypothesis is afforded by the fact, so simply demonstrated by Sir David Brewster, that the corpuscles or filaments which cause the sensation are situated anterior to the retina. Sir David showed, that by looking through a pin-hole at two bright sources of light, such as two lighted candles, we obtain by their two divergent beams, double images of all objects situated within the eye and in front of the retina. By this means, the floating muscae, as we know by frequent repetition of the experiment, are all seen double, depending as they do on corpuscles or filaments anterior to the retina, while any object in or behind the retina, any musca arising from defective sensibility of the retina, or from pressure on its convex surface, could not produce a double image, but would appear single. By means of the same experiment, the size of the corpuscles and filaments, and their distance in front of the retina may also be determined.
Nourishment of the Lens—Pupillary Membrane.—Mr. Nunneley does not formally treat of the development of the organ of vision. The notions he has accidentally emitted on the subject, and especially those on the nourishment of tbe lens in the foetus, and the nature of the pupillary membrane, are in several respects incorrect, as the following extracts will show :—
"In foetal life there is no pupil, the aperture is closed by the membrana pupillaris upon which vessels pass from the iris, forming a plexus not unlike in distribution those upon the posterior and anterior capsules of the lens." (p. 195.)
"The use of the membrane has not been very satisfactorily pointed out; probably it is essential to the due development of the iris in a true plane across the aqueous ohauibers, and serves to keep the base or foundation upon which the mnscnlar and elastic structure are [ii] deposited in a correct form. So long as it exists, the aqueous chamber is of course divided into two. As to the mode of its development there is some difference of opinion, some regarding it as a single membrane, derived altogether from the anterior surface of the iris; but probably the idea of Cloquet is correct; viz., that it is double, the anterior layer being a continuation of a membrane which lines the cornea, and is reflected on the anterior surface of the iris, while a similar mombrane lines the posterior chamber and covers the posterior surface of the iris, the two laminae being at the pnpil in apposition with each other." (p. 196.)
"In the adult, neither vessels nor nerves can be traced in the lens or its capsule; they are therefore regarded as extra-viiscular; but during foetal life, up to the period of birth, and even some little time afterwards, both contain vessels." (p. 259.)
It is plain, that Mr. Nunneley supposes the vessels of the pupillary membrane to be distinct from those covering the anterior capsule of the lens; the pupillary membrane to be destined for the development of the iris; two chambers to exist, each filled with aqueous humour, previously to the breaking up of the pupillary membrane; the pupillary membrane to be a double one, attached to the pupillary edge of the iris; and during foetal life, up to the period of birth, and even some little time afterwards, both lens and capsule to contain vessels; all which suppositions are erroneous. Neither the lens nor its capsule is ever vascular; the pupillary membrane is attached to the front of the iris, leaving the pupillary edge free; the membrane is not double; there is no posterior chamber so long as the pupillary membrane is entire; the pupillary membrane is merely a portion of the anterior half of the vascular sac which surrounds and nourishes the crystalline capsule and lens during the first seven or eight months of foetal life.
In the foetus, the central artery of the retina, entering the eyeball through the optic nerve, divides into two sets of branches; one, which is persistent, to the retina, and another, which is temporary, to the vitreous body, and to the vascular sac which encloses the lens and its proper capsule. The trunk of the latter set, on entering the hyaloid canal, divides mto the central artery of the vitreous body and its circumferential branches. The central artery advances straight to the middle of the hyaloid fossa, and spreads out in radii on the posterior half of the vascular sac. Having reached the edge of the crystalline body, these branches anastomose with the arteries of the zonula ciliaris, these in their turn being connected with the proper vessels of the retina, with the circumferential vessels of the hyaloid, and perhaps also with the ciliary arteries. The radiating branches now turn round the edge of the crystalline body, to be distributed to the anterior half of the vascular sac, which for a time lines the walls of the small aqueous cell, while the comparatively large crystalline is almost in contact with the cornea. The iris being undeveloped, the aqueous cell is not yet divided into an anterior and posterior chamber. When, however, the iris does sprout out, it soon comes into contact with the vascular sac, and adheres to it in such a way that the anterior half of the vascular sac fills the pupil, constituting what is known by the name of the pupillary membrane. The portion of the vascular sac which extends from the pupil to join the posterior half of the sac at the edge of the crystalline, was described by William Hunter,* and is called the capsulo-pupiUary membrane. The iris having joined the vascular sac in the manner now explained, sends vessels into the pupillary membrane to anastomose with its original vessels. These irido-pupillary vessels are larger than those coming from the posterior half of the vascular sac, and anastomose together in arches, turned towards the centre of the pupillary membrane. Continuing to grow, the pupillary or small ring of the iris shoots forth beyond the point where the first formed part, or ciliary ring, joined the vascular sac ; and this m such a way that the circular line of junction comes to be on the anterior surface of the iris, where the ciliary joins the pupillary ring; the edge of the pupil remaining free and unattached.
All this apparatus of vessels for the development of the crystalline is destined to
* Medical Commentaries part 1, second edition, p. 63, note. London, 1777.