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on the retina falls before it, so that the image is confused. I single cause, especially if we bear in mind that the luminous This defect is remedied by means of divergent glasses, which, by removing the rays from their common axis, throw back the focus to the retina. The habit of frequently looking at small objects, and making microscopical observations, has a tendency to bring on short-sightedness. This defeet is common among young people, but diminishes with advancing years.

rays are generally accompanied by calorific rays. But still the identity is not complete, for we know several substances which can be made to give out light in darkness without heat, or at least without any great degree of heat. Bodies of this sort are called phosphorescent, because this property is especially apparent in phosphorus. There are cases in which the phosphorescence is accompanied by a slow chemical action. This is true of phosphorus, certain vegetable or animal substances, as e. g. wood in a state of decomposition, and fish, especially herrings in a putrefied state. In other cases phosphorescence is developed under the influence of a high temperature. For instance, if we heat powdered fluor-spar to about 700 or 900 degrees Fahrenheit, it becomes suddenly luminous and gives forth a bright bluish light. Certain substances become luminous under the influence of solar light. It is thus that the diamond and several other minerals, having been exposed to the rays of the sun, for some time afterwards appear luminous when carried into a dark place. Fluor-spar, diamond, and white marble have the property of acquiring phosphorescence under the action of several successive discharges of a powerful electric battery. Lastly, phosphorescence is very intense in certain insects, especially the glowworm, which varies its light at will..

Long sight is the opposite of what we have just been considering. People having this affection see distant objects very well, but cannot clearly distinguish those that are near. The defect arises from the insufficient convergence of the eye, in consequence of which the image of objects close at hand is formed beyond the retina. But if the objects be removed to a distance, the image will approach the retina; and when they are at a suitable distance, the image is formed exactly on this membrane and the person sees clearly. Long sight is corrected by means of convergent glasses, which draw together the rays before they enter the eye, in consequence of which, if the convergence be suitably chosen, the image is brought exactly to the retina. Till within a few years it was customary to make use of none but bi-convex glasses for long-sighted people, and bi-concave ones for short-sighted people. Wollaston was the first to propose changing these glasses for concave-convex lenses c and F, fig.288, arranged in such a manner that their cur- In many countries, and especially tropical regions, the sea vature corresponds to that of the eye. These glasses, enabling is often covered with a bright phosphorescent light, caused by one to see objects which are far from the optical axis more zoophytes of extreme minuteness. These animalcules shed so clearly, are called periscopic (from TEρiσкоTiw, I look round). subtle a luminous matter, that Messrs. Quoy and Gaimard, Double Sight is an affection of the eye which causes it to see when on a voyage near the equator, having put two of them two objects instead of one. In general, the two images are in a bottle of water, immediately saw the whole mass become almost entirely one over the other, and the one is much more the causes of phosphorescence are the same as those which luminous. Some philosophers, observing that in many cases apparent than the other. Double sight may result from an inequality in the size of two eyes, but it may also affect a develop electricity, and that feeble electric light strongly single eye. This last affection is undoubtedly owing to some resembles that of phosphorescent bodies, infer that phosphordefect in the formation of the crystalline, or other parts of the escence is owing to an electric cause. eye, which causes the luminous pencil to become forked and form two images on the retina instead of one. A single eye may also be affected with triple sight, but in this case the third image is extremely feeble.

Inability to discern Colours.-Some people are unable to distinguish colours, or at least some particular colours. Such persons can discern the outline and form of objects very well, and also the bright and dark portions, but nothing more. One who was thus affected had painted a landscape, in which the earth, the houses, the trees, and the figures were all blue. On being asked by a friend why he had not given each object its proper colour, he answered that he wished to make the colour of the picture accord with that of the furniture in his room; yet this was red. This defect is sometimes called Daltonism, because Dalton, who has carefully described it, was

himself affected with it..

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Various Sources of Light. The various sources of light are the sun, the stars, heat, chemical combinations, phosphorescence, electricity, and meteorological phenomena. The origin of the light emitted by the sun and stars is unknown. We only know that the inflamed substance with which the sun appears to be surrounded, is gaseous, because the light of that luminary, like that which is emitted by gaseous substances when inflamed, affords no trace of polarisation when viewed through instruments constructed for the purpose of detecting polarisation.

As to light developed by heat, according to M. Pouillet, bodies begin to be luminous in darkness at a temperature of about 1350 or 1400 degrees Fahrenheit, and beyond that, the light they emit is brighter in proportion to the increase of heat. It is through the high temperatures accompanying many chemical combinations that these last occasion a disengagement of light. Such is the cause of the artificial light employed to represent lightning, for, as we have seen, flames are nothing more than gaseous substances heated to such a degree as to become luminous.

As bodies become luminous at a high temperature, caloric seems to be then transformed to light, which would favour the idea, that these two agents ought to be referred to one

Action of Light on Vegetables.-Light exercises various kinds of action on vegetables.

1. It is under the influence of the solar rays that the green parts of plants acquire the property of absorbing the carbonic acid in the atmosphere, assimilating the carbon and giving out the oxygen almost pure. In darkness, on the contrary, plants do not give out oxygen, but carbonic acid.

2. It is also under the influence of light that the green substance of vegetables is formed. The less a plant is exposed to some garden cress with green by exposing it to the brightness the light, the paler it becomes. Humboldt slightly tinged of two lamps, and De Candolle obtained a deeper green by using six lamps.

3. The transition from day to night, or from night to day, gives rise to remarkable movements in the leaves and flowers of plants, phenomena popularly described as the sleep and waking of plants. The influence of light is here so apparent, that as soon as the sun appears above the horizon, the sleeping plants awake and open their leaves and petals. By exposing these plants to the artificial light of a sufficient number of lamps, De Candolle succeeded in modifying their habits. He thus saw sensitive plants expand by night, and others open their calixes by day when placed in a dark room.

4. Light also exercises a directing influence upon the branches and roots, the former seeking and the latter avoiding it, at least in many plants. It has been ascertained that all the colours of the spectrum do not exert the same degree of influence upon the branches and roots, but that between blue and violet the greatest intensity of action is produced.

5. Light imparts to vegetables a power of suction. This was demonstrated by the following experiment. Three plants of the same species and the same size were placed in three separate vases containing water, one of which was put in a dark place, the second in a mild light, and the third in the sunshine. The first was found to have imbibed very little water, the second more, and the third by far the most.

DOUBLE REFRACTION.

Double Refraction is a property by which many crystals present two refracted rays for each one incident on their surface, the consequence of which is, that, on looking at an object through these crystals, it appears doubled. Double refraction was first observed by Bartholin, in 1647, but

Huyghens was the first to give a complete theory of it in 1673. Crystals that have the property of double refraction are called double refracting. They are only those which do not belong to the cubical system. Crystallised bodies belonging to this system, and those without crystallisation, as glass, do not possess the property of double refraction, but they may acquire it accidentally by unequal pressure or sudden cooling after having been heated. Liquids and gases are never double refracting. The phenomenon of double refraction is especially apparent in Iceland spar. In quartz or rock crystal double refraction is very feeble, and to render it perceptible requires a crystal of great thickness and suitable apparatus.

Crystals with one Axis.—In a crystal possessing the property of double refraction there are always one or two directions in which only simple refraction is observed, that is to say, only one image of objects is seen. These directions are called optical axes or axes of double refraction. This last name, however is inappropriate, for it is precisely in the direction of these axes that double refraction does not take place. Crystals with one axis are those which present only one direction in which the light does not become forked, and crystals with two axes are those which present two. The crystals with one axis, of which most frequent use is made in optical instruments, are Iceland spar, quartz, and turmalin. Iceland spar has the form of a parallelopiped, the faces of which are inclined at angles of 105 degrees 5 minutes (fig 344). The faces, which are six in

Fig. 344.

number, are rhombi or lozenges, which meet three and three at their obtuse angles at the extremity of a straight line, ab, which is the axis of crystallisation. Brewster has ascertained the general law that in crystals with one axis, the axis of double refraction always coincides with the axis of crystallisation. The principal section of a crystal with one axis is the plane, which passing through the optical axis is perpendicular to a face, whether natural or artificial, of the crystal.

Ordinary and extraordinary Ray-Of the two refracted rays exhibited by crystals with one axis, one always follows the laws of refraction, and is called the ordinary ray.

The other is not generally subject to the laws of refraction, that is to say, the ratio of the sine of the angle of incidence to that of the angle of refraction is not constant, and the plane of refraction does not coincide with that of incidence; hence this ray is called the extraordinary ray.

The images corresponding to these rays are also called ordinary and extraordinary.

Figure 345 shows the cause of the rays in this phenomenon.

Fig. 345.

The parallelogram a bed represents a principal section of a parallelopiped of Iceland spar, which being placed upon a sheet of white paper, we look through it at a black point o, on the paper. The incident ray setting out from the point o, divides into two rays, oi and oe, which being unequally refracted, on emerging convey two images, o' and o", to the eye. If the parallelopiped be turned about on one of its angular points without removing it from the paper, the ordinary image will remain fixed, but the extraordinary image will turn about the other, which shows that the plane of refraction is not in the

411

same position with respect to the plane of incidence, and con-
sequently the extraordinary ray does not conform to the laws
of refraction.
Laws of Double Refraction in Crystals with one Axis.-The
phenomenon of double refraction in crystals with one axis is
subject to the following laws.

always follows the two general laws of simple refraction.
1. The ordinary ray, whatever be the plane of incidence,
nary ray also follows these two laws like the ordinary ray, but
2. In every section perpendicular to the axis, the extraordi-
its index of refraction is not the same as that of this latter;
extraordinary indez.
whence the distinction between the ordinary index and the

follow the second law of refraction, that is to say, the planes
3. In every principal section, the extraordinary ray does not
of incidence and refraction coincide, but the ratio of the sines
of the angles of incidence and refraction is not constant.
the ordinary ray as the extraordinary, the difference of the
4. The velocity of light in a crystal not being the same for
squares of these two velocities is proportional to the square of
the sine of the angle made by the extraordinary ray with the

axis.

This last law is the expression of an empirical formula given by Biot, for connecting together the velocities of the two rays. It also results from formulæ, to which Fresnel was conducted by purely theoretical considerations, which have this remarkable peculiarity, that Biot's formula may be deduced from them. Huyghens, who was the first to give a complete theory of double refraction founded on the system of undulations, discovered a very remarkable geometrical construction, by the aid of which one can determine the position of the refracted ray in all its varied situations with regard to the axis, on knowing its incidence; but his theory was not admitted by philosophers till Malus established its accuracy by numerous experiments.

Laws of Double Refraction in Crystals with two Axes.-Crystals with two axes are very numerous. Of this class are the sulphates of nickel, magnesia, barites, potash, and iron, with sugar, mica, and the topaz of Brazil. In these different crystals, the angles of the two axes assume very different values, varying from three to ninety degrees. Fresnel discovered by theory and demonstrated by experiment, that in the laws of simple refraction, but calling the line which bisects crystals with two axes, neither of the refracted rays follows the angle between the two axes the middle line, and that which bisects the supplement of this angle the supplementary line, he found that in every section perpendicular to the middle line, one of the refracted rays follows the ordinary laws of refraction, and in every section perpendicular to the supplementary line, it is the other ray which follows these laws.

POLARISATION.

Polarisation by Reflection.-Polarisation is a particular modification of the luminous rays, by virtue of which, when once they have been reflected or refracted, they become incapable of further reflection or refraction in certain directions. Polarisation was first discovered in 1810 by Malus, a French philosopher, who died only a few years since. Light is polarised by reflection or refraction. If reflected on a surface of black glass, light is polarised when the reflection takes place at an angle of 35° 25' with the glass. The following are some of the properties of the polarised ray.

1. This ray undergoes no reflection on falling upon a second plate of glass at the same angle of 35° 25′, if the plane of incidence on this second plate is perpendicular to the plane of incidence on the former, but it is more or less reflected if incident at other angles.

only gives one image, if the principal section is parallel or per2. When transmitted through a double refracting prism it pendicular to the plane of incidence, while in every other position with respect to this plane, it gives two images more or less bright.

whose axis of crystallisation is parallel to the plane of inci3. It cannot be transmitted through a plate of turmalin dence, but on the contrary is more and more easily transmitted, as the axis of the turmalin becomes more and more perpendicular to that plane.

All bodies have, like glass, the power of polarising light by reflection, but not all with the same completeness, nor at the same angle of incidence. Marble, for instance, completely polarises light, while diamond and common glass only polarise it partially. Of all bodies metals have the smallest polarising power.

Angle and Plane of Polarisation.-The angle of polarisation of a substance is the angle which the incident ray must make with a level and polished surface of this substance, in order that the reflected ray may be polarised as completely as possible. For water this angle is 37° 15', for glass, 35° 25′, for quartz 32° 28', for diamond 22°, and 33° 30′ for obsidian, a kind of natural black-glass, which polarises light very well. Sir David Brewster has given the following remarkably simple definition of the angle of polarisation: "The angle of polarisation is the angle of incidence for which the reflected ray is perpendicular to the refracted ray." But this definition is not applicable to light reflected by double refracting crystals. In polarisation by reflection, the plane of reflection in which the light is polarised is called the plane of polarisation. This plane coincides with the plane of incidence, and consequently contains the angle of polarisation. It is in this plane that light which has been once reflected cannot be again reflected at the angle of polarisation, in a plane perpendicular to the first. It is also in this plane that it is not transmissible through turmalin whose axis is parallel to that plane. Every ray, therefore, that is polarised by refraction has a plane of polarisation, that is to say, a plane in which it exhibits the phenomena above mentioned.

Polarisation by Simple Refraction.-When a ray of light which is not polarised falls at the angle of polarisation upon a plate of glass with parallel faces, it is only reflected in part, the other part goes through the plate and is refracted, and the light transmitted is partially polarised in a plane perpendicular to the plane of reflection, and consequently to the plane of polarisation of the light which has been polarised by reflection. Arago observed, also, that the reflected and the refracted pencils contained an equal quantity of polarised light, and that the reunion of these two pencils produces natural light. We may therefore regard ordinary light as formed of two equal pencils, polarised at right angles. As a single plate of glass never completely polarises light, we may employ several, one upon the other, and their successive reflections and refractions will give a more complete result. Glasses so placed are called piles, and are often employed to obtain a pencil of polarised light.

Polarisation by Double Refraction.-Light is polarised by double refraction when it passes through a crystal of Iceland spar or any other double refracting substance. The two pencils, which are distinct on emerging, are both polarised completely, but in different planes, which are perfectly or nearly perpendicular to each other. To prove this, we look through a parallelopiped of Iceland spar at a black spot on a sheet of white paper. To the naked eye there appear two images with the same brightness, but if we interpose a plate of turmalin and turn it about in its own plane, each image will disappear and reappear twice for each revolution of the turmalin, which shows that the two emerging rays are polarised in planes perpendicular to each other. The ordinary image vanishes at the moment when the axis of the turmalin is parallel to the principal section of the surface of incidence, and the extraordinary image at the moment when this axis is perpendicular to the same section; whence we infer that the ordinary pencil is polarised in the plane of the principal section, and the extraordinary pencil in a plane perpendicular to that section.

LESSONS IN FRENCH PRONUNCIATION.-No. I.

ALPHABET OF THE FRENCH LANGUAGE. 1. A tolerable Pronunciation of any spoken language may be acquired by imitating the sounds of that language, as uttered by a living teacher. But the READING and WRITING of any language cannot thus be learnt. The pupil must bring into requisition something else besides his imitative powers, if he would thoroughly comprehend any language. The alphabet |

of the language to be learnt must be exhibited, examined, and MASTERED. 2. An Alphabet is a collection of different characters called letters, each of which represents its own peculiar sound. These letters differ from each other in name, form, size, and sound. Used as vehicles of thought, they must not only be familiar to the eye, but their use, both singly and combined, . must be understood.

3. Two objects are to be before the student whilst perusing. these lessons book, viz. :—

First. The acquisition of the correct pronunciation of the various sounds of the letters of the French Alphabet. Secondly. To learn how to combine and use these sounds, in order to read the French Language easily, intelligibly, and profitably.

4. The first object will be accomplished by the aid of analogous English sounds; that is, every sound represented by a letter or combination of letters of the French Alphabet, will be unfolded, analyzed and defined, as far as possible, by means of analogous sounds of a letter or continuation of letters of the English Alphabet. 5. The second object will be accomplished by learning a few brief and simple Rules, illustrated and enforced by appropriate examples.

6. Diligent attention, patient labour, and A DETERMINATION TO SUCCEED, will enable the learner to overcome every obstacle, and thus make him master of a Language not only exceedingly difficult for foreigners to acquire, but beautiful in itself, and coexistent with the triumphs of civilisation. 7. The student's attention is next directed to an inspection of the French Alphabet.

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In the French Alphabet there are only twenty-five letters; it has no letter which corresponds to the English W, though it is occasionally found in French books. It is used only in foreign words, and then pronounced like the English V. 8. The French Alphabet is divided into VowELS and CON9. VOWELS. These six letters are called Vowels, viz. :—

SONANTS.

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11. COMPOUND VOWELS.

They are thus called, because, being united together, each
vowel loses its own simple sound, and helps to form another
new sound. They form but one syllable, and are consequently
pronounced by one emission of the voice.
There are seven Compound Vowels, viz. :—

ai au

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ue and ui

FRENCH ACCENTS.

413

French language, constitutes a marked peculiarity which can17. The constant use of certain marks called ACCENTS in the not escape the attention of the student. Rarely, except in elementary works of the English Language, is the syllable of any given word which requires an emphasis, marked.

18. But it is not so in the French Language; here, accents of various kinds are constantly meeting the eye on every page. One thing, however, must be observed, viz. :-the position of the Accent does not always and infallibly mark the syllable of a nunciation. word, which must receive the stress of voice in common pre

19. Modern Grammarians have established the following rule, viz. TO PLACE THE STRESS OF VOICE ON THE LAST PRONOUNCED SYLLABLE OF EVERY WORD. 20. A slight inspection only of the following examples will

The following ten combinations of THREE SUCCESSIVE VOWELS illustrate the above remarks. are also called Diphthongs, viz. :—

iai

iau ieu oua
uie

oue oui uai

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and

ueu.

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The first syllable of this word is marked with an accent; must the stress of voice therefore be placed upon the syllable DE? No :-if the rule be applied to this word, the stress of voice falls on the last syllable, RER.

:

It will then be asked, What is the use of this accent? We answer, It modifies the sound of the vowel over which it is placed. Again the word used now as an example, has the same kind of an accent as the word used in the previous example had; and also, it is placed over the same vowel. But it has another different accent over the first vowel of the second syllable; and, according to the rule, the stress of voice is not placed either upon the first or second syllable, but upon the last.

They must, however, be pronounced quickly, and as one syllable. Sometimes, also, we find FOUR SUCCESSIVE VOWELS in the Lé-gère-ment same word, viz. :—

ouai oueu ouée

in the word

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jou-ai,
jou-eur, and
bou-ée.

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This second accent (observe its form and position) only Sometimes, however, an accent is placed over a vowel of the serves to modify the sound of the vowel over which it is placed. voice, viz. :-Cé-lé-bri-té. syllable, which, according to the rule, receives the stress of

Bâ-ti-ment

Again, in the word used here as an example, a third, and still different accent is placed over the vowel A. Its presence affects the sound of that vowel only. It has nothing whatever to do understood when applied to words in the English language. with the proper accent of that word, as the term Accent is As a general rule, the stress of voice is not so strong in the French as in the English language.

21. Accents, as used in the French language, are certain marks differing from each other, and placed over certain vowels only, for specific purposes.

22. There are three Accents, viz. :—

called the Acute Accent,
Grave
Circumflex,,

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23. 'The acute accent, is used only over the vowel E, and serves two purposes:

First,-to modify its sound.

Secondly, to mark the existence of a distinct and final syllable, viz. :

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Dé,
Pe-tar-dé,

Trom-pé,
Cér-é-mo-nie.

24. The Grave accent, is used only over the vowels A, E and U, viz. :—

The following combinations of the consonants are called Liquids, viz. :

11 and gn.

Â, Père, and serves two purposes:

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First, to modify the sound of the vowel E.

Secondly, to distinguish one part of speech from another, viz.:

The sounds of these liquids are very common in the French à is a Preposition, a is a Verb, la is an Adverb, la is an Article, language, and will be explained hereafter.

où is an Adverb, ou is a Conjunction.

A

25. This character, which represents the Circumflex words commencing with a vowel or H mute, and is much accent, is the union of the Acute and Grave accents, and is used in the French language, viz. :— placed over each of the Vowels except Y. It indicates that the letter over which it is placed, has a sound twice as long as it has without it, viz. :—

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L'ami instead of Le ami.
L'église
L'homme 66

S'il

La église.

Le homme.
Si il.

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LESSONS IN MORAL SCIENCE. No. I.
CONSCIENCE, OR THE MORAL FACULTY.

As all men, when reason is developed, have a faculty by
which they can discern a difference between objects of sight
which are beautiful and those which are deformed, so all men
possess the power of discerning, a difference between actions,
as to their moral quality. The judgment thus formed is imme-
diate, and has no relation to the usefulness or injuriousness to
human happiness, of the objects contemplated.

Whatever difference of opinion may exist respecting the origin of this faculty, it is universally admitted that men, in all ages and countries, have judged some actions to be good and deserving of approbation, while they have judged others to be bad and of ill desert.

In all languages, we find words expressive of the ideas of moral excellence, and moral evil. In the laws and penalties established in all ages throughout the world, it is evidently implied that some actions ought to be done, and others avoided.

The Hyphen is a short horizontal mark, which is used to In cases of flagrant injustice or ingratitude, all men, of every connect words and syllables, viz. :

A-t-il, Belles-Lettres, Celui-ci, Demi-kilomètre,
Fait-on, Suis-je, and Très-rarement.

Its use in connecting syllables is precisely the same as in the English language; that is, when a word is divided, so that a part of it is at the extreme right-hand of a page, and the rest at the extreme left.

29. APOSTROPHE.

The APOSTROHE is like a comma placed at the upper end of letters, instead of at the lower end, or at the bottom on a line with the lower end.

Its use is, to show the elision or cutting off a vowel before

country and of every age, agree in their judgment of their moral evil. There is, in regard to such actions, no more difference in the judgment of men, than respecting the colour of grass, or the taste of honey. If any man does not perceive grass to be green, or honey to be sweet, we do not thence conclude that men's bodily senses are not similarly constituted, but that the organs of the individual who does not see and taste as other men do, are defective, or depraved by disease.

To determine whether all men have one original moral faculty, the case proposed for their moral judgment should be simply good or evil. For a complex act, in which there is something good and something evil, or rather where there must be an accurate weighing of motives in order to ascertain the quality of the action, is not a proper test as to the existence of a uniformity of moral judgment in men. Therefore,

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