Imatges de pàgina
PDF
EPUB

what to lessen the density. When dry, the refractive power is little inferior to that of crown glass. The second circumstance is, the unequal density of the lens. The ratio of 14 to 13 is founded on the supposition of an equable density: but, the central part being the most dense, the whole acts as a lens of smaller dimensions; and it may be found by Prop. VII. that if the central portion of a sphere be supposed of uniform density, refracting as 21 to 20, to the distance of one half of the radius, and the density of the external parts to decrease gradually, and at the surface to become equal to that of the surrounding medium, the sphere thus constituted will be equal in focal length to a uniform sphere of the same size, with a refraction of 16 to 15 nearly. And the effect will be nearly the same, if the eentral portion be supposed to be smaller than this, but the den. sity to be somewhat greater at the surface than that of the surrounding medium, or to vary more rapidly externally than internally. On the whole, it is probable that the refractive power of the centre of the human crystalline, in its living state, is to that of water nearly as 18 to 17 ; that the water imbibed after death reduces it to the ratio of 21 to 20; but that, on account of the unequable density of the lens, its effect in the eye is equivalent to a refraction of 14 to 13 for its whole size. Dr. Wollaston has ascertained the refraction out of air, into the centre of the recent crystalline of oxen and sheep, to be nearly as 143 to 100; into the centre of the crystalline of fish, and into the dried crystalline of sheep, as 152 to 100. Hence, the refraction of the crystalline of oxen in water should be as 15 to 14: but the human crystalline, when recent, is decidedly less refrac

tivé.

These considerations will explain the inconsistency of different ob. servations on the refractive power of the crystalline; and, in particu. kar, how the refraction which I formerly calculated, from measuring the focal length of the lens, is so much greater than that which is determined by other means. But, for direct experiments, Dr. Wola laston's method is exceedingly accurate.' P.41.

The whole extent of the retina is not equally sensible, nor is its vision equally perfect: the imperfection, Dr. Young remarks, begins within a degree or two of the visual axis, and at the extent of five or six degrees becomes nearly stationary; but, at a still greater distance, is principally, if not wholly extinguished. The imperfection is owing to the unavoidable aberration of the oblique rays, but chiefly to the insensibility of the retina, throughout its whole extent; the sensible portion of which probably coincides with the painted choroïd" of quadrupeds. In general, the retina is of such a form as to receive the most perfect image on every part of its furface that the state of each refratted pencil will admit; and the varying density of the crystalline ren«lers that state capable of delineating such a picture to the greatest advantage. This contrivance is truly a beautiful one, and it is here admirably developed; but, with all the advantage it affords, the eye is seldom perfectly achromatic. Our author next considers more particularly the faculty of the eye of accommodating itself to the different distances of an object, and inquires whether it may arise from the diminution of the radius of the cornea, the increased distance of the lens from the retina, or a change in the figure of the lens itself. We need scarcely observe that Dr. Young decides in favour of the latter cause, in support of which his arguments and experiments are highly judicious and well contrived. We dare not say that the proposition is demonstrated : but, in Dr. Young's hands, it has attained an elucidation and degree of evidence which we scarcely ever expected to have found. He admits, that the action formerly attributed to the external coats cannot afford a solution of the phænomenon. The change must be in the lens itself. He thinks he can trace nerves not wholly into the lens, but very nearly approaching it; and, on the whole, is of opinion that it must be considered as a muscular organ.

• I consider myself as being partly repaid for the labour lost in search of the nerves of the lens, by having acquired a more accurate conception of the nature and situation of the ciliary substance. It had already been observed, that in the hare and in the wolf, the ci. liary processes are not attached to the capsule of the lens ; and if by the ciliary processes we understand those filaments which are seen detached after tearing away the capsule, and consist of ramifying vessels, the observation is equally true of the common quadrupeds, and, I will venture to say, of the human eye. Perhaps this remark has been made by others, but the circumstance is not generally understood. It is so difficult to obtain a distinct view of these bodies undisturbed, that I am partly indebted to accident for having been undeceived respecting them: but having once made the observation, I have learnt to show it in an unquestionable manner. I remove the posterior hemisphere of the sclerotica, or somewhat more, and also as much as possible of the vitreous humour, introduce the point of a pair of scissars into the capsule, turn out the lens, and cut off the greater part of the posterior portion of the capsule and of the rest of the vitreous humour. I next dissect the choroid and uvea from the sclerotica; and, dividing the anterior part of the capsule into segments from its centre, I turn them back upon the ciliary zone. The ciliary processes then appear, covered with their pigment, and perfectly distinct both from the capsule and from the uvea; and the surface of the capsule is seen shining, and evidently natural, close to the base of these substances. I do not deny that the separation between the uvea and the processes extends somewhat further back than the separation between the processes and the capsule ; but the difference is inconsiderable, and in the calf does not annount to above half the length of the detached part. The appearance of the processes is wholly irreconcileable with muscularity; and their being considered as muscles attached to the capsule is therefore doubly inadmissible. Their lateral union with the capsule commences at the base of their posterior smooth surface, and is continued nearly to the point where they are more intimately united with the termia nation of the uvea ; so that, however this portion of the base of the

processes were disposed to contract, it would be much too short to produce any sensible effect. - What their use may be cannot easily be determined : if it were necessary to have any peculiar organs for secretion, we might call them glands for the percolation of the aqueous huinonr; but there is no reason to think them requisite for this purpose.' P. 78.

Some other remarks on the eyes of fishes, insects, &c. are added; but we shall prefer transcribing our author's own recapitulation.

• First, the determination of the refractive power of a variable medium, and its application to the constitution of the crystalline lens. Secondly, the construction of an instrument for ascertaining, upon inspection, the exact focal distance of every eye, and the reinedy for its imperfections. Thirdly, to show the accurate adjusta ment of every part of the eye, for seeing with distinctness the greatest possible extent of objects at the sam: instant. Fourthly, to measure the collective dispersion of coloured rays.in the eye. Fifthly, by immerging the eye in water, to demonstrate that its accommodation does not depend on any change in the curvature of the cornea. Sixthly, by confining the eye at the extremities of its axis, to prove that no material alteration of its length can take place. Seventhly, to examine what inference can be drawn from the experiments hitherto made on persons deprived of the lens ; to pursue the inquiry, on the principles suggested by Dr. Porterfield; and to confirm his opi, nion of the utter inability of such persons to change the refractive state of the organ. Eighthly, to deduce, from the aberration of the lateral rays, a decisive argument in favour of a change in the figure of the crystalline ; to ascertain, from the quantity of this aberration, the form into which the lens appears to be thrown in my own eye, and the mode by which the change must be produced in that of every other person. And I Aatter myself that I shall not be deemed too precipitate in denominating this series of experiments, satisfactorily demonstrative.' P. 82.

* III. On the necessary Truth of certain Conclusions obtained by Means of imaginary Quantities. By Robert Woodhouse, A.M. Fellow of Caius College. Communicated by the Rev. S. Vince, A.M. Plumian Professor of Astronoiny in the University of Cambridge.

. We have read with pleasure this ingenious defence of mathematics against those who reproach its paradoxes, and ridicule its impossible quantities, which are admitted as means of attaining veracity. We cannot however give any account of it which will satisfy the reader who has not made mathematical inquiries the object of his attention; and a mathematician can only read it with advantage in the work itself.

IV. On the Production of artificial Cold by Means of Muriate of Lime. By Mr. Richard Walker, Coinmunicated by Heitry Cavendish, Esq. F.R.S.

We have already noticed M. Lowitz's very singular discovery of the great cold produced by adding muriate of lime te snow or pounded ice. Three parts of the former to two of the latter sunk the thermometer to go, and quicksilver in large quantities was frozen by this experiment. Mr. Walker has succeeded in repeating the experiment, and, by operating on a mixture previously cooled, has produced a temperature so low as 91-perhaps the lowest degree ever procured by human art. He then tried the muriate of lime, prepared so as to continue solid during the summer, and produced, by five parts of the salt to four of water, 29 degrees of cold. At any time therefore, in the summer, water may be reduced to 21 degrees of Fahrenheit, 11 degrees below the freezing point; and by employing ice thus procured, with some necessary precautions, mercury may be frozen in the hottest summer day. A view of the general effects of the different frigorific mixtures is added, and a postscript, announcing that ice formed on the outside of a vessel containing the frigorific mixture is transparent, while that produced from a fluid immersed in the mixture is usually opaque. The solution is, that it is frozen more slowly; but Mr. Walker generally throws an air of importance over common observations. ,

• V. Account of a monstrous Lamb. In a Letter from Mr. Anthony Carlisle to the Right Honourable Sir Joseph Banks, Bart. K. B. P.R.S.

This lamb was an extraordinary production, but by no means singular. It wanted wholly the cerebrum and the head. It had two external ears, and the remains or rudiments of the small bones of the ear between these organs. One passage led from the external parts to the oesophagus and larynx, The cerebellum was perfect. --The following remarks are judicious and philosophical. We could, we think, did our limits permit, pursue them with some success.

- The narration of these appearances assists and confirms other facts, in demonstrating that the formation and growth of animals in the uterus are independent of any influence from those parts of their brain which properly belong to sensation. We have to regret, that this animal did not live to show the phænomena of volitions directed to its limbs, and other apparatus, without that intelligence from the organs of the senses which regulates and directs the efforts of perfect animals. The careful observance of such circumstances may in future bring lis to discoveries of the highest value, in that part of physiology which is now enveloped in deep mystery: the facts at present collated are not sufficient. The intellectual phæno. mena of persons who sustain known injuries of particular parts of the brain; the appearances on the dissection of ideots, with their mental particularities; the anatomical history of maniacs-all promise, when properly cultivated, a series of truths, which it may not be

extravagant to hope will open sublime views into those recesses of our construction which justly rank among the most curious, if not the most important objects of research. P. 142.

"VI. An anatomical Description of a male Rhinoceros. By Mr. H. Leigh Thomas, Surgeon. Communicated by George Fordyce, M.D. F.R.S.

The description before us is in a great degree new, at least to the English reader; for there is more than one, dissection of the rhinoceros described by French anatomists, though, if our recollection serve, the accounts are not so full as in the present article. The animal died of an inflammation in the lungs; and, when alive, showed no attachment even to the person who fed it, but continued torpid, and apparently stupid. The anatomy, in general, corresponded to that of the horse. The cæcum was however much larger, and the internal parts of the stomach lined with a secreting membrane. The intestines; were short, but their surface greatly increased by long processes, resembling the valyulæ conniventes. The liver was very soft, but this might have arisen from an unnatural mode of living. The urinary organs were apparently very simple in their structure, as an injection passed into the ureter, though urged with little force. From the convex side of the penis being towards the body, this animal must. have been a retrocoïent.

. Whilst the animal was living, the eyes always appeared dull and watery; the upper and under palpebre were scarcely ever observed to come together; the palpebra tertia was frequently carried over the cornea, and corresponded in shape and structure to that of the ox. The muscles of the eye-ball were exactly similar to those of other graminivorous animals: the globe of the eye was not larger. than that of the sheep; and the cornea was much smaller. Upon cutting through the sclerotic coat, it was found somewhat harder and thicker than what is observed in the sheep; and, upon endeavouring to separate it from the choroid, I found an uncommon resistance at the posterior part of the eye; though in other parts the adhesion between the coats appeared less than what takes place. in the human body. This unusual connexion naturally directed my attention more particularly towards it; when I readily discovered four processes, arising by distinct tendons from the internal and posterior portion of the sclerotica, and at cqual distances from the optic nerve. These processes passed forwards between the coats, gradually becoming broader, and being insensibly lost in, and form. ing a part of, the choroid, at the broadest diameter of the eye; the connexion between the coats around the outer circle of the cornea was the same as is observed in the eye of other animals. The processes had a muscular appearance-the fibres running forward in a radiated direction; they were detached from the coats with the greatest facility, except at their origins and insertions, where it required considerable force to tear them from the sclerotica; and, at,

« AnteriorContinua »