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Of ly diffused over the body of an animal is that of touch Sensation, or feeling. Animals that possess scarcely any other sense seem always to have that of touch. It is doubtless by this that polypi, actinia, and other water animals, perceive the approach of their prey, or are warned of impending danger, from the agitation of the water that is communicated to their bodies. Indeed so general is this sense, that some physiologists think we may reduce all others to it as a genus; and suppose that smelling, tasting, hearing, and seeing, are only species of feeling. This reference is not uncommon in ordinary speech, as it is not unusual to talk of feeling a smell.

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By touch, taken in its ordinary limited sense, we perceive the more striking external qualities of bodies, as figure, hardness, softness, roughness, smoothness, moisture, dryness, heat, cold; of all which, except figure, we could scarcely form any idea by the other senses. There is probably no sense that can so well supply the place of others as that of touch; and it is particularly acute in those who have lost their sight or hearing. See the article BLIND, especially the Appendix.

The organs of touch are the skin and its productions, or rather the nervous papillæ (see ANATOMY, N° 76.) that form so large a part of the true skin. As many animals, however, have the body so enveloped in a scaly, shelly, or hairy covering, as to prevent the actual contact of the body by external objects, there are other organs that seem destined to fulfil this office. In man, the points of the fingers and the lips are the most delicate feeling organs; in many quadrupeds too, the lips seem to possess an exquisite sensibility, and in some, as the rhinoceros, the upper lip is lengthened out as if to serve the purpose of a hand. The prolonged snouts of the tapir, the shrew, the mole, and the hog, seem to answer the same purpose; and the exquisite sensibility and flexibility of the trunk of the elephant is well known to fit that organ for almost all the purposes to which the human hand can be applied. The tail, in some species of monkey, opposum, and ant-eater, and in some reptiles, seems to possess a high degree of sensibility. In some animals, as the cat, the whiskers are employed as organs of feeling, as we know that these are erected when the animal is passing through a narrow hole. Several species of fishes have cirri and tentacula, which they seem to use as fingers in ascertaining the approach of their prey; and in insects, the antenna and the palpi are evidently organs of feeling, as are the arms, the tufts, and tentacula of sea-stars, sea-urchins, actiniæ, medusa, and many zoophytes.

Most of the actions of external bodies on the surface. of the animal body, are merely mechanical, though the sensations which they communicate may often be the effects of a chemical change in some of the feeling organs, and this change can be produced only in consequence of the power of simple pressure, to form or destroy some of the combinations that take place in the animal system. The sensations which appear most evidently to arise from a chemical change in the organs, are those that give notice of a change of temperature. When a body that has a temperature below that of the animal, comes in contact with the surface of this latter, we know that it abstracts from that surface a part of its caloric, as by the contact it gradually acquires the temperature of the animal; unless, indeed, it be so large and so cold as altogether to destroy life. As, however,

the resistance which the animal body gives to a too Of great change of temperature, generally confines this Sensation. change to the surface of the body; there must be something more than a mechanical or a chemical action, or the sense of feeling must depend chiefly on the vital principle.

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As the sense of feeling, from its general diffusion, Universali may be considered as the most essential of all the ty of feelsenses; its degrees of perfection have considerable in-ing. fluence on the nature of different animals.

Of all vertebral animals, man seems to possess this sense in the most perfect degree; but among the invertebral animals, the touch seems to improve as the other senses degenerate; and those animals which appear to have no other sense, possess this in so exquisite a degree that they seem to feel even the light.

Dr Darwin thinks it probable, that the animal body is furnished with a distinct set of nerves for the sensa tion of heat and cold. We do not see the necessity of this, as we think that this sensation is very naturally reducible to that of feeling.

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mal body.

To this head naturally belongs the consideration of Sensibility what parts of the human body possess sensibility, and of the ani what are insensible. This discussion is curious, and some time ago exercised the ingenuity of two very able physiologists, Haller and Whytt; between whom it gave rise to a long and warm dispute. We cannot pretend to enter into the merits of this controversy, for an account of which we refer our readers to Dr Whytt's Physiological Essays, and to the Principes de Physiologie of Dumas, tom. ii. part iii. sect. 1. chap. 1.

The general result seems to be, that many parts will appear sensible or insensible, according to the nature of the stimuli applied to them, and that many of those parts which in their natural and healthy state appear insusceptible of pain, are when inflamed or otherwise altered by diseases, highly sensible; and that the brain, which is considered as the centre of all sensation, and the puncture or laceration of which is attended with most distressing symptoms in other parts, is to ordinary stimuli as insensible as the cuticle or the nails. See also on this subject, Bichat "Anatomie Generale," tom. i. p. 161-167.

The principal morbid affections of this sense, are pain, itching, and want of feeling; for an account of which see MEDICINE, N° 77. The functions of the skin, independently of its use as an organ of touch, will be considered in two of our succeeding chapters.

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Of be most susceptible of the taste of hellebore, the palate Sensation. of belladonna, and the gullet of wormwood.

86 On what

tion of taste depends.

The momordica elaterium is said chiefly to affect the back of the tongue, and colocynth its middle.

The greater or less perfection of this sense depends the perfec- much on the softness, flexibility, and moistness of these parts. As man seems to possess these qualifications in a more eminent degree than most other animals, so, in the natural unsophisticated state of the tongue, he probably enjoys the benefit of taste much more highly than they. Such is the case with all young children, and with the peasant, whose simple fare appears to be eaten with a much greater relish than all the delicacies of the voluptuary, who must have recourse to various stimuli to enable him to derive gratification from even the daintiest viands (A).

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Taste seems to be more exquisite when the sapid body is strongly pressed between the tongue and the palate. Taste is also rendered more acute when the tongue is stimulated by various condiments, as pepper, mustard, which even, when not taken in such quantity as to be very perceptible themselves, evidently increase the relish of the dishes which they season. Much also depends on the nature and state of the bodies that are applied to the organs of taste. These must, in the first place, be either fluid, or capable of solution in the saliva. They must also possess some saline or acrid quality, to render them capable of acting on the nervous papilla. It was formerly supposed, that saline bodies alone possessed the power of affecting the organs of taste; and it was conceived by Bellini, that the different flavours of saline bodies depended on the figure of their crystalline particles. M. Dumas has taken considerable pains, and has advanced several arguments, to show the absurdity of this hypothesis; and we think has treated it with more seriousness than it deserves. That the different sensations which sapid bodies excite in our organs of taste, depend chiefly on a difference in their chemical nature, must, we think, be allowed, and some have gone so far as to suppose, that the sensation depends on some chemical affinity between the sapid body and the nervous fluid. The impression which sapid bodies make on the orof taste is modified by age, sex, temperament, and gans habit. We know that children are particularly pleased with sweet things, while high seasoned dishes and vinous liquors are more palatable to people of a more advanced age. Women, from various causes, especially during pregnancy, and when labouring under hysteric affections, have often very singular tastes. People of a warm and a mobile constitution are often affected by flavours that are almost insensible to others; and custom will render palatable many substances, which, when first tasted, are rejected with disgust.

Besides the gratification afforded to animals by the sense of taste, this is supposed to afford one of the principal means of distinguishing between wholesome and deleterious substances. Indeed, with respect to the

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inferior animals, this discriminating sense is seldom known to fail, and in this instance, they are superior to Sensation. man, who is often deceived. There are many poisonous herbs, the fruits or roots of which have a taste not unpleasant, but which cannot be eaten with impunity. On the morbid affections of taste, see MEDICINE, N° 48.

3. Of Smelling.

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The sense of smelling, like that of taste, is nearly al- Smelling.. lied to feeling, and is one of those by which we become acquainted with the mechanical and chemical properties of external bodies. It is caused by volatile particles flying off from odorous bodies, and diffused or dissolved in the atmosphere, in union with which they enter the nostrils and affect the nerves of the smelling organs.

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It is difficult to ascertain what are the essential or- Organs. gans of smelling. We know that in most animals which breathe through lungs or gills, there is either a nose, or there are certain holes that serve the purpose of nostrils; but in many animals there is nothing similar to these, and yet there is every reason to believe that they possess the sense of smelling in an exquisite degree.

Insects discover their food at a distance. Butterflies seek their females, even when inclosed in boxes; and as they are liable to be deceived by resemblance of colour, it is evident that these insects are guided in many circumstances by the sense of smell. Thus the flesh-fly (musca vomitoria) lays its eggs on plants that have a foetid smell, imagining that it places them on corrupted flesh, and the larvae which are thus produced perish for want of their necessary food.

As the organ of smell, in all animals which respire air, is situated at the entrance of the organs of respiration, the most probable conjecture that has been proposed respecting its seat in insects, is that of Baster, since revived by several naturalists, who placed it in the mouths of the tracheæ or air tubes. Besides many other reasons that might be stated in support of this opinion, we may observe, that the internal membrane of the trachea appears very well calculated to perform this office, being soft and moistened, and that the insects in which the tracheæ enlarge, and form numerous or considerable vesicles, are those which seem to possess the most perfect sense of smelling. Such are all the scarabai, the bees, flies, &c.

The antennæ, which other anatomists have supposed to be the seat of smelling in insects, do not appear to Cuvier to possess any of the requisites for that organ.

The mollusca, which respire air, may also possess this > sensation at the entrance of their pulmonary vessels; but it is not necessary to search for a particular organ of this sense in them, as their whole skin appears to resemble a pituitary membrane. It is everywhere soft, fungous, and is always moistened by a great quantity of muscous matter. Finally, it is supplied with numerous nerves, which animate every point of its surface.

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(A) It is generally supposed, that the sense of tasting is more acute in some of the inferior animals than in man; an opinion which is founded chiefly on the greater size and number of the papilla of the tongue in those animals. It is scarcely possible to decide this point; but we should conceive, from the infinite variety of substances that are occasionally subjected to the human palate, and from the extreme delicacy of taste displayed by some individuals, that man has the advantage of his brute neighbours in this sense.

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The worms and soft zoophytes, and all the polypes, Sensation. are probably in the same situation. It cannot be doubted but that these animals enjoy the sense of smell. It is chiefly by it that they discover their food, particularly the species that have no eyes. Aristotle remarked, that certain herbs, which have a strong odour, were avoided by cuttle-fishes and the octopus.

91 Nature of

understood.

Of all the substances which affect our organs of sensaodours little tion, odours are the least understood, though the impressions which they make on the animal body appear to be most powerful and extensive. Some bodies are always odorous, because the whole or a part of their substance, being volatile, it is constantly flying off: others become odorous, only under certain circumstances; as when a body containing a volatile principle in its composition is decomposed by another that has a less affinity for that principle, e. gr. when muriate of ammonia is decomposed by quicklime.

Odours seem to be propagated in the air, much in the same manner as one fluid is diffused through another. Their motion is not direct like that of light, nor is it rapid or susceptible of reflection and refraction like light and caloric. The odorous particles of volatile bodies may enter into combination with different substances, by chemical affinity, and thus lose their original properties. In this way the effluvia of putrid meat are destroyed by fresh burnt charcoal, and the noxious exhalations from pestilential apartments are removed by the vapours of nitric or muriatic acid.

These circumstances seem to prove that each smell is occasioned by a particular substance floating in the atmosphere. There are others, however, which appear to indicate that odour is not always produced in this man

ner.

Several bodies yield a strong smell for a great length of time, without sustaining any sensible loss of substance; such, for example, is musk. Some odours are perceived when no evaporation can be observed, as the smell which arises from the friction of copper, that produced by the fusion of a great number of bodies, and even by the melting of common ice. In other cases, real evaporations produce no sensible odour; this may be remarked on the disengagement of several gases, and even on the ordinary evaporation of water. Perhaps these phenomena prove only that the force of sensation is not proportional to the quantity of the substance by which it is excited, but that it depends on the nature and degree of Cuvier. the affinity of that substance with the nervous fluid*. The action of the greater part of odorous substances on the nervous system, is rendered manifest by a number of other effects besides the sensation of smell; some produce faintings, others giddiness, or even convulsions. Some, on the contrary, serve to remove these disorders: indeed the greater part of medicines act in general rather by their volatile and odorous parts, than by their other principles; and afford new proofs of the influence exercised in the animal economy by the gaseous and impalpable substances, the greater part of which are doubtless still unknown to us.

We know not whether odours have a peculiar vehicle, besides the matter of heat, which is common to them all in their quality of vapours or elastic fluids. We cannot explain why odours are agreeable or disagreeable to us, nor why those that are disgusting to us appear pleasing to other animals, and vice versa. Though

man and other animals are generally pleased with the Of odour of those substances which serve them as food; yet Sensation. when their appetite is satisfied, this odour often becomes displeasing to them. On the contrary, some animals appear to have a passionate fondness for strong smelling substances which seem altogether useless to them. Thus cats are extremely fond of cat-mint, and the fresh roots of valerian. In general, those odours which are most disagreeable indicate that the substances from which they proceed are injurious. Thus venomous plants, putrid flesh, and poisonous minerals, have commonly an unpleasant odour. This rule, however, is not universal; and the sense of smell, like that of taste, is not an unerring guide to man, whatever it may be to other animals.

It appears that the effluvia of odorous bodies are capable of diffusing themselves through water as well as air; for when these substances are thrown into water as bait for fish, we find that these animals are attracted by the smell from a considerable distance.

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The comparative physiology of this sense is very curi- Compara ous, though we cannot explain the reason of the differ- tive physioences that we find to take place in the various tribes of logy of animals. Man in a state of civilized society, where he smelling. may have recourse to a great variety of means by which to distinguish the properties of bodies, has less occasion for acuteness of smell; but we know that savages are in that respect greatly his superiors. Their smell is so acute, that like a blood hound, they can scent their enemy to a great distance, and pursue his track with almost certain success. Among birds and beasts of prey we also find that acuteness of smell is a very general property. Hyænas, wolves, vultures, and ravens, can distinguish the putrid carcases on which they feed many miles off; and it is asserted by naturalists, that jackalls hunt in packs, and follow their game like hounds by the scent. There is a curious diversity in this respect among birds, some having this sense very acute, others very blunt. We are told by Gattoni*, that the cock is scarce- * Scarpa de ly affected with the smell of ammonia or hartshorn, Auditu et while the duck is said to avoid all powerful odours Olfactu. whether agreeable or otherwise. We are not sufficiently acquainted with the nature of the olfactory membrane, nor with that of the nerves distributed to it, to enable us to form an opinion respecting the degree and the kind of sensations they procure to different animals. It may, however, be at first sight presumed, that all things in other respects being equal, the animals in which the olfactory membrane is most extensive, enjoy the sensation of smell most exquisitely; and experience confirms this conjecture. It would be curious to learn why the animals which possess the sense of smell in the highest degree, are precisely those which feed on the most fetid substances, as we observe in dogs which eat carrion.

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The sense of hearing is more important than any Hearing. which we have yet noticed, but it appears to be less generally diffused.

By means of it we become acquainted with those properties of bodies which fit them for making sensible impressions on the air, as hardness, elasticity, &c.; and these impressions on the air, when communicated to he organs of hearing, convey to our mind the ideas of sou: d.

Of

By this sense we derive two of the highest gratificaSensation. tions that we are capable of enjoying, viz. the pleasures of conversation and of music; and in this way most animals hold intercourse with each other.

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Organs.

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Varieties of sound.

The organs of hearing differ exceedingly in the various classes of animals. The human ear and its appendages have been described in ANATOMY, Part I. chap. vi. sect. 4.; and for an account of these organs in other animals, we must refer to Cuvier's Lectures, vol. ii. or the Comparative Anatomy of Blumenbach, chap. xx. Red-blooded animals without exception have evident auditory organs; and analogous parts are found in many of the white-blooded. In a great number of the inferior classes, however, no such parts have been ascertained, though it is certain that many of them do really hear. In all those in which these organs have been detected, there is always found a gelatinous pulp, covered with a fine, elastic membrane, and in this pulp the ramifications of the auditory nerve are lost. It is therefore, highly probable that the seat of hearing resides in the minute nervous fibres that are distributed through the pulp, and that this latter is the medium by which sounds are communicated from the percussed air. We may form a tolerably just idea of the manner in which this pulpy substance is connected with the external movements that are the cause of sound; for this quivering jelly will readily receive the concussions of the air or water that are transmitted to it from the vibrations of sonorous bodies, and communicate them to the nervous filaments. Thus far only can we trace the motion of sound; but the steps by which this motion is carried on till the perception of sound is produced in the mind, are equally unknown to the anatomist and the metaphysician.

The philosophy of sound has already been treated of under ACOUSTICS. It is necessary here to remark only, that the qualities of sound may be distinguished into force, depending on the extent of the vibrations of the body from which the sound proceeds; tone, depending on the velocity of the vibrations; resonance, arising from the intimate composition of the sonorous body; simple modulation of voice, and articulations.

96 ComparaThe human ear can distinguish all these different quative physio-lities with relation to one sound; this distinction is made logy of with wonderful accuracy, by persons who frequently exhearing. ercise that faculty, and particularly by professional musicians. The other mammalia exhibit proofs that they are capable of distinguishing the qualities of sound which relate to speech, that is to say, simple vocal modulations and articulations: for we may observe daily, that they remember the sound and signification of several words. Some are strongly affected by certain sounds. Acute tones produce a painful sensation in dogs, and we also observe that these animals are terrified by violent noises; they therefore distinguish these two properties. Birds have a feeling, no less exquisite, of voice, tone, articubation, and even resonance, since they learn to sing with great correctness; and when their vocal organs permit them, can completely counterfeit the human speech, with all the modifications practised by the individuals they imitate.

Ås to cold-blooded animals, it is well known that several of them call each other by certain sounds, and that others, which are incapable of producing sounds, can at least understand them, as carps, which appear when the

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noise of a bell indicates to them that they are to be fed, &c.; but we know not what qualities of sound they Sensation. distinguish, and how far, in this respect, the delicacy of their sense of hearing extends *. For the morbid affections of hearing, see MEDICINE vol ii. sect. N° 85.

5. Of Seeing.

*Cuvier

Comp. Anat.

13.

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As we ascend from the simpler to the more complex Seeing. senses, we find a greater scope for description and observation; but we also find our physiological difficulties increased. The sense of touch being the most simple of all the senses, requires but a simple organization, and is the most widely diffused; that of vision, on the other hand, is the most complex, and requires for its mechanism, a more elaborate set of organs. There is not, in the whole animal structure, a more curious and admirable organ than the eye, whether we contemplate it in its most perfect state in the human body, or in its most simple conformation, as it appears in the horn of a snail. The anatomy of the human eye has been sufficiently Organs. described in the article ANATOMY, Part I. chap. vi. sect. 5.; and if our readers desire a fuller account of this organ, we may refer them to the elegant work of Professor Soemmering. The structure of the eye in the inferior animals is well described in Cuvier's twelfth lecture, and in Blumenbach's Comparative Anatomy, chap. xvi. We shall extract from the former a description of the eyes of insects and crustaceous animals, as being among the most curious and least known subjects of comparative anatomy.

"The structure of the eye of insects is so very different from that of other animals, even the mollusca, that it would be difficult to believe it an organ of sight, had not experiments, purposely made, demonstrated its use. If we cut out, or cover with opaque matter, the eye of the dragon fly, it will strike against walls in its flight. If we cover the compound eyes of the wasp, it ascends perpendicularly in the air, until it completely disappears; if we cover its simple eyes only, it will not attempt to fly, but will remain perfectly immoveable.

"The surface of a compound eye, when viewed by the microscope, exhibits an innumerable multitude of hexagonal facets, slightly convex, and separated from one another by small furrows, which frequently contain fine hairs, more or less long.

"These facets form altogether a hard-and elastic membrane, which, when freed of the substances that adhere to it posteriorly, is very transparent.

"Each of these small surfaces may be considered either as a cornea, or a crystalline; for it is convex externally, and concave internally, but thicker in the middle than at the edges, it is also the only transpa- · rent part in this singular eye.

"Immediately behind this transparent membrane there is an opaque substance, which varies greatly as to colour in different species, and which sometimes forms, even in the same eye, spots or bands of different colours. Its consistence is the same as that of the pigment of the choroides; it entirely covers the posterior part of the transparent facets, without leaving any aperture for the passage of the light.

"Behind this pigment we find some very short white filaments, in the form of hexagonal prisms, situated close to each other, like the stones of a pavement, and

precisely

98

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precisely equal in number to the facets of the cornea ; Sensation. each penetrates into the hollow part of one of these facets, and is separated from it only by the pigment mentioned above. If these filaments are nervous, as in my opinion they appear to be, we may consider tach as the retina of the surface behind which it is placed but it will always remain to be explained, how the light can act on this retina, through a coat of opaque pig

Cuvier's

ment.

"This multitude of filaments, perpendicular to the cornea, have behind them a membrane which serves them all as a base, and which is consequently nearly parallel to the cornea; this membrane is very fine, and of a blackish colour, which is not caused by a pigment, but extends to its most intimate texture; we observe in it very fine whitish lines, which are trachea, and will produce still finer branches, that penetrate between the hexagonal filaments, as far as the cornea. By analogy, we may name this membrane the choroides.

"A thin expansion of the optic nerve is applied to the posterior part of the choroides. This is a real nervous membrane, perfectly similar to the retina of redblooded animals; it appears that the white filaments, which form the particular retine of the different ocular surfaces, are productions of this general retina, which perforates the membrane I bave named choroides, by a multitude of small and almost imperceptible holes.

"To obtain a distinct view of all these parts, it is necessary to cut off the head of an insect that has the eyes large, and dissect it posteriorly; each part will then be removed in an order the reverse of that in which I have described them.

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"In the cray fishes, in general, the eye is situated on a moveable tubercle. The extremity, which is rounded on every side, and sometimes elongated into a cone, when viewed by a glass, presents the same surfaces as the eyes of insects. When we cut this tubercle longitudinally, we observe that the optic nerve passes through it in a cylindrical canal, which occupies the place of its axis. Årrived at the centre of the concavity of the eye, it forms a small button, which detaches very fine filaments in every direction; at a certain distance these filaments meet the choroides, which is nearly concentrical with the cornea, and covers the spherical brush of the extremity of the nerve, like a hood. All the distance between the choroides and the cornea is occu>pied, as in insects, by white filaments, closely arranged in a perpendicular direction to each other, and which have the extremity next the cornea also coated with a black pigment.

"These filaments perforate the choroides, and are Lectures, continuations of those produced by the button, which terminates the optic nerve."

vol. ii. 99 Immediate

The immediate seat of vision is still in dispute; but seat of vi- it appears to be the expansion of the optic nerve upon sion proba the inner coat of the eye. The other parts of that orbly the re- gan serve to collect, refract, absorb, and sometimes even - reflect, the rays of light, according as these operations are required for the distinct vision of any particular animal. Those animals that seek for their prey during

tina.

night, have a pupil that is very dilatable, and have Of very little of that dark substance called pigmentum ni- Sensation. grum, that lies between the retina and the choroid coat in diurnal animals. Thus, the former have their eyes better adapted to receive and to retain the feeble rays of light, and thus possess a great advantage over the animals which they pursue, whose eyes are calculated for seeing best in a strong light.

The subject of vision has been so fully considered under OPTICS, Part I. sect. 5. that it is unnecessary for us to give any detailed account of it here. We shall therefore merely enumerate the principal phenomena.

sion.

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1. The rays of light proceeding from luminous bo- Phenome dies, are collected by the cornea; variously refracted by na of vithe aqueous, crystalline, and vitreous humours, till they meet in a point (in perfect vision) in the retina, from which the sensation conveyed to the brain, excites there the ideas of light, colour, and other qualities of extreme objects, of which the eye is capable of judging.

2. The image of the object thus pictured on the reti na, is inverted, though the mind is habituated to perceive it as if it were erect.

3. There is a certain point within the eye where the retina is deficient, and here the luminous rays make no impression.

4. The eye is calculated to see objects most distinctly at certain distances or foci, though these distances vary considerably in different species, and different individuals. A person of ordinary sight can read a middle-sized print most distinctly at the distance of about eight inches. Those who require a less distance are near-sighted, or myopes, and in them the point of divergence of rays is before the retina. Those who require a greater distance are long-sighted, or presbyopes; and in these the point of divergence is behind the retina.

5. In those animals that have two eyes, an image of a luminous object is formed in each, though the mind is accustomed to unite both images into one. In strabismus or squinting, the two eyes not being similarly directed, do not concur in producing a single object.

6. Though the images of many objects are impressed on the retina at the same time, the mind can attend distinctly to only one of them.

7. In perfect vision, the pupil contracts or dilates according to the greater or less quantity of light that is present.

8. When the eye has looked steadily for some time on a circumscribed space, of a particular colour, as a piece of red paper placed on a white ground, it perceives a border of a different colour surrounding the original spot. This surrounding colour is called the accidental colour of the former, and differs according to the colour of the original spot. In the present instance it is green, or bluish-green. The other natural colours are attended by the following accidental colours, viz. ORANGE, by blue, with nearly an equal proportion of indigo; YELLow by indigo, with a mixture of violet; GREEN by violet, with a mixture of red; BLUE by red, with a mixture of orange; INDIGO by yellow, with a considerable mixture of orange; and VIOLET by green, with a considerable mixture of blue (B).

The

(B) Dr Darwin, in his Zoonomia, vol. i. sect. 2. employs the phenomena of accidental colours to prove that the

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