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the size of the pupil varying inversely as the intensity of the ON PHYSICS, OR NATURAL PHILOSOPHY. light. The iris also serves to correct the spherical aberration
by preventing the marginal rays from crossing the edge of the No. LII.
crystalline. (Continued from page 379.)
The Aqueous Humour.–Between the posterior portion of
the cornea and the anterior of the crystalline, is a transparent THE EYE CONSIDERED AS AN OPTICAL
liquid called the aqueous humour. The space e, occupied by
this humour, is divided into two compartments by the iris ; INSTRUMENT.
the part b, between the cornea and the iris, is called the Structure of the Human Eye.—The eye is the organ of vision, anterior chamber; the part o, between the iris and the crystalthat is to say, of the phenomenon by virtue of which the line, is called the posterior chamber. light emitted or reflected by bodies, produces in us the sensa
The Crystalline is a transparent body, with the form and protion that reveals their presence to us. It is placed in a bony perties of a double convex lens, placed behind the iris and cavity or socket, and supported in its position by means of very near that membrane. It is enveloped in a transparent muscles, tendons, and nerves, which render it capable of a membrane called the capsule, which adheres at its edge to great variety and delicacy of movements, while the eyelids the annular band formed by the ciliary processes (or hair-like and eye brows protect it from external injury. Its size is projections), 9. ncarly the same in all individuals, the apparent differences of The anterior surface of the crystalline is less convex size being caused by the smaller or larger opening of the than the posterior surface. Its tissue is composed of a series lid.
of layers almost concentric, which are harder in the centre Fig. 338 represents a transverse section of the eye from the than at the circumference. The outermost coats have almost
a liquid softness. The refracting power of these coats decreases Fig. 338.
from the centre to the circumference.
The Vitreous Humour and Hyaloid Membrane. -The transparent mass, resembling the white of an egg, which occupies the whole of that portion h of the eyeball which is behind the crystalline, bears the name of the vitreous humour or vitreous body. It is enveloped in the hyaloid membrane, which
covers the posterior surface of the crystalline capsule, and all b
the internal surface of another membrane called the retina.
The Retina and Optic Nerve.-The retina m is a membrane intended to receive the impression of the light and transmit it to the brain, by means of a nerve n called the optic nerve, which, proceeding from the brain, enters the eye and spreads over the retina in the shape of a nervous net-work.
The retina and the optic nerve have no other special function than that of receiving and transmitting to the brain impressions of images. They are altogether insensible to injury from external objects. They have been cut and pricked with
out appearing to cause any pain to the animals upon which front to the back. As appears from that figure, the general
these experiments have been performed. form of the eye is that of a spheroid, the curvature of which
The Choroid Membrane k, is a membrane interposed between is greater in front than behind. The eye is composed of the the retina and the sclerotic. It is essentially vascular, and following membranes and media—the cornea,
the iris, the covered on its internal surface with a dark substance like the pupil, the aqueous humour, the crystalline, the hyaloid and pigment of the negro's skin. Its object is to absorb all the choroid membranes, the retina and the optic nerve.
rays that are not required to produce distinct vision. The The cornea, a, is a transparent membrane situated in choroid membrane is fringed with a series of hair-like projecfront of the ball of the eye.
It has the appearance of a small tions 9, called, as we have stated above, ciliary processes, spherical cup, with a base of nearly half an inch in diameter. between the iris and the crystalline capsule, to which they Its circumference is so closely attached to the sclerotic or adhere, forming round it a disk like that of a radiate flower. outermost tunic, that some anatomists consider them as one By its vascular tissue the choroid membrane serves to convey and the same membrane.
the blood to the interior of the eye, and especially to the The Sclerotic, i, is a membrane which, with the cornea,
ciliary processes. envelops all the constituent parts of the eye. In front it has Course of Rays in the Eye.-Considering the various parts of an opening, nearly circular, in which the cornea is encased; which the eye is composed, it may be conepared to a camera the posterior and internal portion is perforated to afford a obscura, of which the pupil is the aperture, the crystalline passage for the optic nerve.
the converging lens, and the retina the screen on which the The Iris, d, is an annular opaque diaphragm or partition, image is portrayed. The effect is therefore the same as that fastened at its outer circumference, and free at its central edge. produced at one conjugate focus of a bi-convex lens by an This membrane is placed between the cornea and the crystal object placed at the other focus. Let A B, fig. 339, be an line. It is this that forms the coloured part of the eye. object placed in front of the eye, and
let us consider the rays It is pierced by an opening called the pupil, which in man is emitted from any point a in the ojbect. Of all the rays procircular. In the lower animals it assumes various forms. In ceeding from it, only those which are directed towards the those of the feline tribe, it is narrow and elongated vertically, pupil penetrate the eye and contribute to vision. These rays, direction. It is by the pupil that the rays of light penetrate tion, which makes them approach the
axis A a, drawn through into the eye. Its diameter, which varies in the same indivi- the optical centre of the crystalline. When they meet the dual, is between one-tenth and
of an inch, but these crystalline they are again refracted as by a bi-convex lens. At may occasionally be surpassed. The alternate enlarge- last, after having undergone further refraction in the vitreous ment and contraction of the pupil are very rapidly performed, humour, they meet in the point a, and there form the image they are constantly going on, and play an important part in of the point a. The rays which proceed from the point B go Hhenomenon of vision. The pupil contracts under the to form the image of this point at 0.
Similar remarks apply influence of strong light, and dilates in a feeble one. The to all the intermediate points between A and B, whence we movements of the iris appear to be involuntary;
see that a very small real but reversed image ab of the object From what has been stated, it will be seen that the iris is a A B is formed upon the retina when the eye is properly conscreen with an opening of variable magnitude, and its function stituted. is that of regulating the quantity of light which enters the eye, Reversing of Images.—To prove that the images formed on VOL. V.
the retina are really reversed, the eye of an Albino or white this angle increases or decreases in proportion to the size negro is employed, because in this singular race the choroid of the object, and for the same object it decreases in promembrane is without pigment, and consequently the light can portion as the distance increases, which is seen in the accompass completely through it. The eye is stripped of the cellular panying figure, 341. The consequence is, that the more disiissue which envelops it at the back, and placed at an aper- tant objects are, the smaller they appear, for the secondture in the window-shutter of a dark chamber, when by the ary axes AO BO crossing each other in the centre of the aid of a magnifying lens it may be clearly seen that the images crystalline, the size of the image projected on the retina on the retina are reversed.
depends on that of the visual angle A o B. The reversing of images in the eye has much occupied the Estimation of Distance and the Size of Objects.--The estimation attention of philosophers and physiologists, and numerous of distance and size depends on the concurrence of several theories have been put forward to explain how it is that we do circumstances; the visual angle, the optical angle, comparison not see objects reversed. Some consider that it is by custom | with objects whose size is familiar to us, and the diminution
and a sort of education of the eye that we see objects rectified, of the clearness of the image by the interposition of an atmo-
same manner, although very distant mountains may be seen Optical Axis, Optical Angle, Visual Angle.The principal under a very small angle, and occupy only an insignificant optical axis of the eye is the axis of the whole ball, that is to space in the field of vision, yet, as we are accustomed to aerial say, the straight line with regard to which the figure is sym-/ perspective, we do not overlook or mistake their real great. metrical. In a well-formed eye, it is the straight line passing ness. The optical angle is also an essential element in estithrough the centre of the pupil and the crystalline, as oo, fig. mating distance. This angle, increasing or diminishing as 339. The lines A a, B b which are apparently but not really objects approach or recede, the motion that we give our eyes, straight, are secondary axes.
It is in the direction of the in order that their optical axes may converge towards the priucipal optical axis that objects are most clearly seen. object which we are looking at, suggests an idea of its distance.
The optical angle is the angle B A C, tig. 340, formed by the | However, it is only by long custom that we are enabled thus
principal optical axes of two eyes directed towards the same to determine a relation between the distance of objects and the object. This angle is smaller in proportion to the distance corresponding motion of the eyes. It is well known that those of the object.
who have been born blind and afterwards obtained sight by The visual angle is the angle A o B, fig. 341, under which an the operation of cataract, at first think all objects equally object is seen, that is to say, the angle formed by the secondary | distant. axes drawn from the optical centre of the crystalline to the Distance of distinct vision. The distance at which objects opposite extremities of the object. For the same distance must be placed in order to be seen with the greatest clearness,
is called the distance of distinct vision. This distance varies Gassendi maintained, that at any one instant perception takes
Taylor and Woollaston contend that two points symmetri.
two eyes directions a o and Bo (fig. 342), we see only one
Others are of opinion that the diameter of the eye, from the bine to produce a single sensation.
object recedes. Hunter and Young attributed a contracting | superficial extent. It is probable that the solidity of objects property to the crystalline, by virtue of which it assumes a could not be perceived with one eye, were it not otherwise more or less convex form, in such a manner as to make the known to us. In fact, when the object is at a small distance, rays always converge on the retina.
and we look at it with both eyes, as the two axes must conKepler, Camper, and many others , considered that by the verge towards it, the perspective'is altered for the two eyes
, getion of the ciliary processes, the crystalline is capable
of and the images are perceptibly unequal. This is easy to removing and approaching the retina more or less. Others prove by looking alternately at the same object with each eye. different distances, not to any movement of the retina or images that the perception of relief appears to result, as is er Yetalline so as to make them approach or recede from each proved by the following experiment. other
, but to the fact that the variations in the focal distance The Siercoscope.-Mr. Wheatstone has invented an ingenious of the crystalline humour caused by the increasing distance of apparatus, called the stereoscope, which
serves to show the of the image. This last theory is confirmed by the experi- solidity. This apparatus, as modified by Sir David
Brewster, ments of Magendie and De Haldat. Sturm has also pro- consists of a small wooden box, the upper compartment of pounded a theory not very dissimilar, according to which the which contains two tubes for directing the optical axes. and it will be sufficiently condensed to produce the sensation separately through a convergent lens placed in each tube. of clear vision. Consequently, when external objects approach These drawings represent the same object but with different of recede
, it is only necessary that the retina be always between perspectives which are precisely those that would correspond these two foci, or nearly coincide with one of them, in order to the optical axis of each eye, if it looked at the object at a that the image may be distinct.
short distance. Hence in looking down the tubes each eye Bingle sight with two eyes. When the two eyes are fixed receives the same impression as it it looked
at the object itselt, upon the same object, an image is formed upon each retina, and the result is so distinct and vivid a perception of reliet and yet we see only one object. To explain this phenomenon, that the illusion is complete and truly surprising.
Br the aid of the stereoscope, M. Foucault and Dr. piano, and an empty room is the oli? machnery required
. Jer she
Say thirty or forty combine and arrange to give a pennt: sege with two parcis of complementary colours, and looking give the weekly sum of six stwings and eightpence
, and BET &: each ruloured dibe with each eye, we see only a single white that an employer, or alternately the friends oi the young people
, cibe wijck protes that the sensation of white light may arise wonld spare a room, and the cost of ligbi and small extra from two chrumetic complementary impressions made simul- was put at eightpence weekly, surely in any large town • tantulity upon the two tetinas,
dancing master might be ioni who for this fee would give the needful instruction in the steps, in the figure of the ordinary quadrille, in the minor a cour (a capital old dance in respect to teaching graceraness, the curtsey, the wall, and
the manner of entering and counting a room. No DOTS. DO FEMALE EDUCATION-X. IV. men, ought or should be admired into the room during these
| lessons; this should be made as strict s law as it was with By 6:LYELPEX.
the ancient priestesses of Greece whilst they performed the
religious rives of the sacred tetrples. Laexo ad esercise calculated to form and improve the
This method of teaching failing. let the Female Dancing 2-sboruc te m2. a branch of the education of all classes, School be managed thus. Wee the weekly conmibutions aze v. I in, I as crian, when education is better understood have reached a fair sum, let t=3 of the elder and most receta. in I: IL V, 14 22. prout. I dasay many good folks of the girls be selected from the rest, and sent to take lessons
E syte that young int and women, in mills, of some good dancing master or mistress for three or four w**, sure, za to www bow dance. walk, and months. In that time they ought to acquire suficient
M & r * quis my yar, genueman or lady knowledge to enable them, at its ciose, to become teachers to ** Vratnih mri, metih in my deliberate the rest; and the weekly sum paid by those they teach would Vista, x125", 7. syempre", *k2 this human serve to give them some slight remuneration, and to per ist 6. Anse * very writ. *** meet.ce; and piano or violin payer, and the bire of a roon it neediale
. * **** ****ys one of us Those thus taught could in their turn teach, and thus the W ? ', , ** . wpepe 2 * 7me. And Dancing School would be seif-supplying to its own needs W.70*.
**** *****! Wayak. saus tut be taken care, however, being taken that the teachers were gracefal vh. ******* MUL SUO 644** r muraire-honte and efficient. des, *WW** *4 wt, At;!-egymand, or w . -, * * you la...A .** :* mure than dances and fashionable steps may be left to those who have
This will be as far as it is needful to go. Fashionable ZI: VALI CORSA Unama ** *.17 4 *****, and is time to acquire them. I only ask as much practice and Home scene! It must be death, di it wil be done knowledge as will enable young women, if their empleyers A www.1 Ta era in the wrist bany of this ununtry is give a holiday dance to join in it with ease and gracefulness OMT? *** tis euexy/1, tunnetu waking, man; and to open to them an occasional source of innocent and rational Why tor is in the past having area, in the to teach them
to walk as women ought to walk, with grace and College and wher, its use mus he will govem partly in love, defect is almost a proverb; but this I am sure is more oning
ease. So few English women walk well, that our feminine party auxvereny--30Te Wikiy tian bugentore, do so with grace and in the dignity of mantum: Why shall his words and to a bad manner than to any physical deficiency. earnest thoruzni me ball their with through a coarse, To this amount of dancing I would add a careful and conprovincial, awkward, vulgur mann? Why shall not he, whó tinuous practice of what are called Calisthenic Exercises to morally has nothing w lear from any man,' say what he has to improve the figure. I need not enter here into the matter, as bay to statesman or w noble with respect for respect to it will have attention in the Popular Educator. In the mestothers is one of the best partuma of our own self-respect-yet while, however, great advance in these matters might be made, unabashed and fearlonely? And why shall not woman in het is, whilst practising a slow marching walk with the foot home sow the seeds of a toute for and an appreciation of beauty lifted up and the step made from toe to heel, a heary book, in the minds and habits of her little children, through comely or other proportionate weight were carried on the headmanners and acts which in due time shall show their fruit in the arms, in the meanwhile, hanging gracefully down. This new development of the plastic, txule, hctile, mechanical arts should be practised till the weight can be carried for a con: at which they labour Yet, thene things must and will be siderable time without falling of moring--the spise in such effected. What is spiritual and aristocratic in culture is case, in order to balance the weight, assuming a most erect destined to change the whole aspect of common life. And and graceful posture. It was party owing to their habit being so, let me point out what is first neediul amongst the of carrying jars of water on their heads, that the women of people—what are some of the first su.ps in the cultivation of ancient Greece were so renowned for extraordinary grace
fulness ; and at this day the women of many parts of the The little which has yet been done in this direction is rather to East, of the mountainous districts of Spain and Italy, and of be lamented than otherwise. The “Dancing Schools of the Connemara in Ireland, owe to this same custom much of their manufacturing towns, where both sexes herd together not an exquisite loveliness of figure and morement. I have myaelf much for rational amusement and improvement aus for more woon & rustic Welch woman carry a pail of milk up a mountain sensual purposes, cannot be but baneful, and in connexion with the grace of a goddess.
penny theatres," protested against as one of the main causes of juvenile delinquency and crime. What I mean in phynical culture of the body is, that it in a degree necessitates
What is, however, best amongst other noble effects of this In those departments of social education where the means conceive that any young woman who has thus learnt to domicilio of culture are so terribly deficient
, I have long wanted the who walką well, who practises the same simple means which some one of them take the initiatory step, and teach a "clanu
or race of sculptor the world
has ever known, can be otherwise these young women in her father or husband's
warehouse or thun modellerine demeanout and gentle in spect. I cannot wenty or thirty girls would be invaluable. A tiddle, or n / and hailed womo male companion from the opposite side of the