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be of the same substance, the refracting angle ord of the The Simple Microscope.-We distinguish two sorts of micro-
second being smaller than the refracting angle BC F of the scopes, the simple and the compound. The simple microscope
first, the two prisms will produce the same effect as one prism is sometimes formed by a single convergent lens, sometimes
BAP; that is, the white light which passes through them will by several lenses superposed, which act as a single one. We
not only be bent, but decomposed. But if the first prism BCF, have already seen that in the simple microscope, or magnifying
be made of crown glass, and the second of flint, we can destroy glass, the object observed is placed between the lens and its
the dispersion, while preserving the refraction. The flint principal focus, and that then the image is virtual, erect, and
being more dispersive than the crown, and the dispersion magnified.
produced by a prism diminishing with the angle of refraction Different dispositions are given to the simple microscope.
in the prism, it follows that in suitably diminishing the angle Fig. 311 represents that which has been adopted by M
of refraction cpd in the flint prism, with relation to the angle
of refraction bcr in the crown prism, we can render the dis.

Fig. 311.
persive power of these two prisms equal; and as from their
position the dispersion occurs in opposite directions, it is
compensated,--that is, the emergent rays Eo are obviously
reduced to a parallelism, and consequently give white light.

The relation of the angles B C F and crd, however, which
bring to a parallelism red and violet rays, not having the
same effect on the intermediate colours, it follows that with
two prisms we can in reality achromatise only two rays of the
spectrum. In order to obtain perfect achromatism, it is
necessary to have seven prisms, of substances unequally dis-
persive, and whose angles of refraction shall be suitably deter-
mined.

As to refraction, it is not corrected at the same time as dis-
persion, for in order to this it is necessary that the refractive
power of bodies should vary, as Newton supposed, in the same
proportion as their dispersive power, which is not the fact.
Consequently, the emergent ray Bo, does not issue parallel to
the incident ray s 1, and there is a deviation without sensible
decomposition.

Achromatic lenses are formed of two lenses made of sub-
stances unequally dispersive, the one A in flint, is concave-con-
vex-divergent, fig. 306; the other B, in crown glass, is bi-convex,
and one of its surfaces must coincide exactly with the surface
of the first. In lenses, as well as in prisms, it is necessary to Raspail. A horizontal support, which can be elevated or
have seven glasses to obtain perfect achromatism; but in depressed by means of a rack, and in which we see, at point
óptical instruments two are sufficient, as they give the neces- D, a dark eye-hole, in the centre of which is placed a lens more
sary curve to achromatise red rays and yellow rays.

or less convex. Beyond this is the holder B, which is fixed, Absorption of Light by Transparent Media.---We know no and on which, between two glass plates c, is placed the object substance which is perfectly transparent. Glass, water, air to be viewed. As it is necessary that the object should be even, gradually tinge the light which passes through them, strongly illuminated, we receive the light diffused through and with sufficient density these media will weaken it, so that the atmosphere, on a concave glass reflector m, which is init cannot act on the retina. We observe, for example, that a clined in such a way that the reflected rays fall upon the great number of stars, that are invisible on the plain let the object. In order to use this microscope, the eye must be sky be ever so clear, become visible when we ascend a high placed very near the lens, which is elevated towards the mountain.

object, or lowered, till we find the position in which the This gradual loss experienced by light in passing through image is seen with most distinctness. With a simple microtransparent media is called absorption ; and the cause of it is, scope we can obtain a very distinct magnifying power to the reflection which it undergoes upon the molecules of trans- 120 times its diameter. The magnitude may be determined parent bodies. If all the simple rays were equally transmis- by calculation, or by experience, with the aid of the microsible through transparent substances, the latter would be meter, which will be described by-and-by. colourless. Now this is never the case ; for transparent bodies Compound Microscope.-— The compound microscope, when reallow certain luminous rays to pass more easily than others. duced to its greatest degree of simplicity, is formed of two It is for this reason that, under great density, the air appears lenticular convergent glasses, the one named objective, because blue, and a plate of thick glass is green. Glass coloured red it is turned towards the object; the other, which is less con. by protoxide of copper suffers only red rays to pass, and vergent, is called ocular, because it is nearer the eye of the absorbs all the others, even when it is not thick. It is by the observer. effect of absorption that the light of the sun is less intense, Fig. 312 represents the progress of the luminous rays, and when that luminary is at the horizon, than when it is in the zenith, for then the density of the atmosphere is much more

Fig. 312. considerable.

OPTICAL INSTRUMENTS. Various kinds of Optical Instruments. We give the name of optical instruments to combinations of lenses, or of lenses and mirrors, which may be divided into three groups, according to the uses for which they are designed. 1. Instruments whose sole object is to magnify the images of objects too small for the naked eye: these are microscopes. 2. Instruments used for observing the stars or distant objects : these are the formation of the image in the compound microscope. An telescopes. 3. Instruments designed to produce on a screen object

, A B, being placed very near the principal focus of the images, diminished or magnified, which may be useful in the objective, m, but a little beyond, with relation to this glass, art of design: these are the illuminated chamber, the

dark an image, a'b, real, inverted, and greatly amplified, is formed chamber, the daguerreotype, the magic lantern, the fantasma- at the other side of the objective. Now the distance of goria, the megascope, the solar microscope, and the gas microscope. the two glasses m and x is such that the place of the The two first groups give only virtual images, and the last only image a bois formed between the ocular, n, and its focus, real images, except the illuminated chamber.

F. It follows from this that to the eye placed at e, which

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beholds this image with the ocular, the last glass produces at the distance at which the eye sees the image of the micro-
the effect of the simple microscope, and substitutes for the meter.
image ab a second image a'b', which is virtual, and magni- The Compound Microscope of Amici.-We have hitherto de-
fied anew. This second image, erect in relation to the first scribed only the principle of the compound microscope. Our
image, is inverted in relation to the object. We may say, attention will now be directed to the principal accessories of
then, in regard to this last analysis, that the compound micro- this apparatus. Invented in 1620, it has from time to time
scope is nothing but the simple microscope, applied not to the received a number of improvements. The most important,
object

, but to its image already magnified by the first lens. however, date but thirty years back, and are due chiefly to
Magnitude-the Micrometer.---The magnitude in every optical M. Amici, in Italy, and M. Ch. Chevalier, in France.
instrument, is the relation of the absolute size of the image to
that of its object. Magnitude in the compound microscope is

Fig. 313.
the product of the respective magnitudes of the objective and
the ocular; that is, if the first of these glasses magnify 20
times, and the other 10, the definitive magnitude will be 200.
Magnitude depends on the greater or less convexity of the
objective and the ocular. It has reached 1,500 in diameter,
and even more ; but then the image loses in distinctness what
it gains in magnitude. In order to obtain images perfectly
clear and distinct, the magnitude should not exceed 500 or
600 in diameter, which gives on the surface an image 250 to
360 thousand times greater than the object.

Magnitude may be measured experinientally by means of
the micrometer-a small plate of glass, on which are traced
with a diamond parallel lines, to or idö of a millimetre asun-
der. The micrometer is placed before the objective; then, Fig. 314 'represents in its essential parts the microscope
instead of receiving directly on the eye the rays which emerge known as the Microscope of Amici, or the Microscope of Ch.
from the ocular o, we receive them on a plate of glass with Chevalier. In the ancient microscopes, the tube was always
parallel faces a (fig. 313), having an inclination of 45°, and we vertical, and the lenses were not achromatic. Amici was the
place the eye above so as to see the lines of the micrometer first who adopted a disposition which allows the tube to be
formed by reflection upon a scale divided into millimetres, horizontal or vertical at pleasure, and it was Chevalier who,
which is traced on the screen E. Counting the number of in 1823, first applied achromatic lenses to the microscope.
divisions on the scale, which correspond to the number of Our diagram represents the microscope in the horizontal posi-
lines of the image, we thence deduce the magnitude. For tion, which is generally less fatiguing to the eye. But it may
example, if the image occupy upon the scale 45 millimetres also be placed vertically. For this purpose, the elbow G is
and comprehend 15 lines of the micrometer-supposing that elevated, and the large tube A, which bears the ocular, rises
the interval of these be ide of a millimetre-the absolute size to its place upon the objective B: Again, the microscope may
of the object will be 1%, and that of the image being 45 milli- have an inclined position. To give it this, we draw the bolt m,
metres, the magnitude will be the quotient of 45 by 10% or which secures the lower part of the apparatus, and we make
300. There is in this process, however, a source of error the whole move on a hinge d, which binds the microscope to a
which results from the difficulty of placing the screen exactly cylindrical colụmn serving for its support.

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On a rectangular trunk, parallel to this column, is the object- / of ascertaining the different kinds of vegetable matter,
bearer B, which can be raised or lowered, by means of a small to detect the adulterations too often introduced into flour,
projection which fits into a rack, and which is moved by chocolate, etc., and the presence of cotton, or wool, or silk,
means of the button D. The object which we wish to observe etc. in the fabrics of the loom.
is placed between two glass plates c, situated on the object.
bearer. A glass concave reflector , receives the light diffused
through the atmosphere, and reflects it on the object, which is
thus powerfully illuminated--an indispensable condition on
account of its magnitude. The object-bearer is perforated at LESSONS IN READING AND ELOCUTION.
its centre with a circular opening, which is perceived through
the plates B, and which is designed for the passage of the light

No. XIX.
conveyed by the reflector.
The fig. 316 shows the position of the glasses, and the march

EXERCISES ON EXPRESSIVE TONE-continued,

POETRY.
Fig. 315.

[Marked for Inflections.)
We believe that poetry, far from injuring society, is one of the
great instruments of its refinement and exaltation. It lifts the
mind above ordinary life, gives it a rèspite from deprèssing
cares, and awakens the consciousness of its affinity with what
is púre and noble. In its legítimate and highest efforts, it has
the same tendency and aim with Christianity; that is, to
spiritualise our nature. True, poetry has been made the
instrument of vice, the pander of båd pássions; but when
genius thus stóops, it dims its fires, and parts with múch of
its power; and even when Poetry is enslaved to licéntiousness
and misanthropy, she cannot wholly forgét her trúe vocàtion.
Strains of pure fèeling, touches of tenderness, images of
innocent happiness, sympathies with what is good in our

nature, bursts of scorn or indignation at the hollowness of the
M

world, passages true to our mòral nature, often escape in an
immoral work, and show us how hard it is for a gifted spírit to
divorce itself whòlly from what is good.

Poetry has a natural alliance with our bést affections. It
delights in the beauty and sublimity of outward nature and of
the soul. It indeed portrays with terrible energy the excèsses

of the pássions, but they are passions which show a mighty of the rays. The objective z is formed of one, two, or three nature, which are full of power, which command áwe, and achromatic lenses, as represented at k, the principal focal dis- excite a deep though shuddering sỹmpathy. Its great tendency tances being from 8 to 10 millimetres. The ocular a H is and purpose, is, to carry the mind beyond and above the formed of one simple lens, or two lenses, A and x, these being beaten, dusty, weary walks of ordinary life; to lift it into a achromatic or not. It is easy to follow the march of the purer element, and to breathe into it more profound and luminous rays. After being reflected on the mirror m, they generous emotion. It reveals to us the loveliness of nature, meet towards the object, and thence they are directed towards brings back the freshness of youthful fèeling, revives the the objective. After traversing that, they encounter a rectan- relish of simple pleasures, keeps unquenched the enthusiasm gular prism P, in crystal, on the hypothenuse of which they which warmed the spring-time of our being, refines youthful undergo a total reflection; then, taking the direction of the love, strengthens our interest in human nature, by vivid tube G A, the rays fall upon the lens y, and form, beyond, a delineations of its tènderest and lóftiest feelings, spreads our real and magnified image of the object c. The last lens acts sympathies over all classes of society, knits us by new ties as a simple microscope, substituting for the first image a with universal bèing, and, through the brightness of its second image vertical and again magnified,

prophetic visions, helps fàith to lay hold on the future life. The intermediate image u has for its object to collect the We are aware that is it objected to poetry, that it gives wrong too oblique rays which do not fall upon the ocular A. It views, and excites fàlse expectations of life, peoples the mind enlarges the field of the microscope, while rendering the image with shadows and illusions, and builds up imagination on the smaller and more distinct. This glass serves also to correct ruins of wisdom. That there is a wisdom against which the defect of achromatism, which the objective more or less poetry wars,-the wisdom of the senses, which makes physical presents. As to the aberration of sphericity, it is corrected comfort and gratification the supreme good, and wealth the by the screens m and n (fig. 315), which intercept the rays chief interest of life,—we do not dený: nor do we deem it the that tend to cross the lenses too near the edges. In order to least service which poetry renders to mankind, that it redeems extinguish all interior reflection, which could mar the distinct them from the thraldom of this earthborn prudence. ness of the images, the sides of the tube are blackened inter- But, passing over this topic, we would observe, that the nally,

complaint against poetry as abounding in illùsion and decépIf the object be transparent, we illuminate it by means of tion is, in the main, groundless. In many poems there is a reflector placed under the object-bearer; if it be opaque, we more of truth, than in many histories and philosophic theories. use the lens L, supported by the object-bearer, which concen- The fictions of génius are often the vehicles of the sublimest trates the light upon the object.

rèrities, and its flashes often open nèw regions of thought, and Finally, the apparatus has several spare oculars and objec- throw new light on the mysteries of our bèing. In poetry the tives, by which the magnitude may be augmented or dimi- lètter is falsehood, but the spirit is often profoundest wisdom. nished.' We can also diminish the magnitude by suppressing And if truth thus dwells in the boldest fictions of the poet, much one or even two of the lenses of the objective.

more may it be expected in his delinéations of life; for the The microscope has been the cause of the most interesting present life, which is the first stage of the immortal mind, coveries in botany, in zoology, and in physiology. Ani- abounds in the materials of poetry, and it is the highest office

whose existence had been altogether unknown, have of the bard to detect this divine element, among the grosser observed in vinegar, in water, in the paste of flour, in pleasures and labours of our earthly being. d fruits, and in certain kinds of cheese; while the circula- The present life is not wholly prosaic, precise, tàme, * and un and the globules of the blood have become visible. The aicroscope is also susceptible of numerous applications in

* A negatire sentence, ending with a rising inflection, has the falling the industrial arts. For example, it furnishes the means bilde on sis penultimate word or Clause,

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CAUSES OF WAR.

finite. To the gifted eye it abounds in the poètic. The affec-work; they make education, the soul's nutriment, cheap; tions which spread beyond ourselves, and stretch far into they bring up remote and shrinking talent into the cheerful futùrity; the workings of mighty pàssions, which seem to arm field of competition : in a thousand ways they provide an the soul with an almost superhuman energy; the innocent audience for lips, which nature has touched with persuasion; and irrepressible joy of infancy; the bloom and buoyancy, and they put a lyre into the hands of genius ; they bestow on all dazzling hopes of youth; the throbbings of the heart when it who deserve it, or seek it, the only patronage worth having, first wakes to love, and dreams of a happiness too vást for the only patronage that ever struck out a spark of " celestial èarth; wòman, with her beauty, and grace, and gentleness, fire,”-the patronage of fair opportunity.

This is a day of improved education; new systems of and fulness of feeling, and depth of affection, and her blushes teaching are devised; modes of instruction, choice of studies, of purity, and the tones and looks which only a móther's heart adaptation of text-books, the whole machinery of means, have can inspire ;-these are all poetical.

been brought, in our day, under severe revision. But were I It is not true that the poet paints a life which does not

to attempt to point out the most efficacious and comprehenexist. He only extracts and concentrates, as it were, life's sive improvement in education, the engine, by which the ethéreal èssence, arrests and condènses its volatile fràgrance, greatest portion of mind could be brought and kept under brings together its scattered beauties, and prolòngs its more cultivation, the discipline which would reach furthest, sink refined but evanescent joys; and in this he does well; for it deepest, and cause the word of instruction not to spread over is good to feel that life is not wholly usurped by cares for the surface, like an artificial hue, carefully laid on, but to subsistence and physical gratifications, but admits, in measures penetrate to the heart and soul of its objects, --it would be which may be indefinitely enlarged, sèntiments and delights popular institutions. Give the people an object in promoting worthy of a higher being. -Channing.

education, and the best methods will infallibly be suggested
by that instinctive ingenuity of our nature, which provides
means of great and precious ends. Give the people an object

in promoting education, and the worn hand of labour will be [To be marked for Inflections by the student.]

opened to the last farthing, that its children may enjoy means What are sufficient causes of war let no man say, let no denied to itself.-E. Everett. legislator say, until the question of war is directly and inevitably before him. Jurists may be permitted, with comparative

SUCCESS OF THE GOSPEL. safety, to pile tome upon tome of interminable disquisition upon the motives, reasons, and causes of just and unjust war.

[To be marked for Inflections by the student.] Metaphysicians may be suffered with impunity to spin

The assumption that the cause of Christianity is declining, the thread of their speculations until it is attenuated to a is utterly gratuitous. We think it not difficult to prove that cobweb; but for a body created for the government of a great the distinctive principles we so much venerate, never swayed nation, and for the adjustment and protection of its infinitely so powerful an influence over the destinies of the human race diversified interests, it is worse than folly to speculate upon as at this very moment. Point us to those nations of thethe causes of war, until the great question shall be presented earth, to which moral and intellectual cultivation, inexhaustifor immediate action,- until they shall hold the united ques- ble resources, progress in arts, and sagacity in council, have tion of cause, motive, and present expediency, in the very assigned the highest rank in political importance; and you palm of their hands, War is a tremendous evil. Come when point us to nations whose religious opinions are most closely it will, unless it shall come in the necessary defence of our allied to those we cherish. Besides, when was there a period, national security, or of that honour under whose protection since the days of the Apostles, in which so many converts national security reposes, it will come too soon,--too soon for have been made to these principles, as have been made, both our national prosperity,

-too soon for our individual happi- from Christian and pagan nations, within the last five-andness,--too soon for the frugal, industrious, and virtuous habits twenty years ? Never did the people of the saints of the Most of our citizens,—too soon, perhaps, for our most precious High look so much like going forth in serious earnest, to take institutions. The man who, for any cause, save the sacred possession of the kingdom and dominion, and the greatness cause of public security, which makes all wars defensive, -the of the kingdom under the whole heaven, as at this very man who, for any cause but this, shall promote or compel this

day. final and terrible resort, assumes a responsibility second to

But suppose the cause did seem declining, we should see no none, nay, transcendantly deeper and higher than any, which

reason to relax our exertions, for Jesus Christ has said, Preach man can assume before his fellow-man, or in the presence of the gospel to every creature; and appearances, whether prosGod, his Creator.-Binney.

perous or adverse, alter not the obligation to obey a positive

command of Almighty God. Again, suppose all that is FOUNDATION OF NATIONAL CHARACTER.

affirmed were true. If it must be, let it be. Let the dark (To be marked for Inflections by the student.)

cloud of infidelity overspread Europe, cross the ocean, and

cover our beloved land, -let nation after nation swerve from Mental energy has been equally diffused by sterner levellers the faith,—let iniquity abound, and the love of many wax than ever marched in the van of a revolution,—the nature of cold, even until there is on the face of this earth but one pure man and the providence of God. Native character, strength, church of our Lord and Saviour Jesus Christ, -all we ask is, and quickness of mind, are not of the number of distinctions that we may be members of that one church. God grant that and accomplishments, that human institutions can monopolize we may throw ourselves into this “ Thermopylæ of the moral within a city's walls. In quiet times, they remain and perish universe.' in the obscurity to which à false organization of society con- But even then, we should have no fear that the church of signs them. In dangerous, convulsed, and trying times, they God would be exterminated. We would call to remembrance spring up in the fields, in the village hamlets, and on the the years of the right hand of the Most High. We would mountain tops, and teach the surprised favourites of human recollect there was once a time, when the whole church of law, that bright eyes, skilful hands, quick perceptions, firm Christ, not only could be, but actually was, gathered with one purpose, and brave hearts, are not the exclusive appanage of accord in one place. It was then that that place was shaken,

as with a mighty rushing wind, and they were all filled with Our popular institutions are favourable to intellectual im- the Holy Ghost. That same day, three thousand were added provement, because their foundation is in dear nature. They to the Lord. Soon we hear, they have filled Jerusalem with do not consign the greater part of the social frame to their doctrine. The church has commenced her march:-torpidity and mortification. They send out a vital nerve to Samaria has, with one accord, believed the gospel ; Antioch every member of the community, by which its talent and has become obedient to the faith; the name of Christ has been power

, great or small, are brought into living conjunction and proclaimed throughout Asia Minor ; the temples of the gods, strong, sympathy with the kindred intellect of the nation; and as though smitten by an invisible hand, are deserted; the enery impression on every part vibrates, with electric rapidity, citizens of Ephesus cry out in despair, Great is Diana of the through the whole. They encourage nature to perfect her Ephesians ; licentious Corinth is purified by the preaching

courts,

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