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Sri of the Spectrum.-The various colours of the solar
are not continuous. For several degrees of refrang
by day are wanting altogether. The result of this is, that
the whole extent of the spectrum there are a great
ale of very narrow dark stripes, which are named the
drum, and which were first observed by Wolls
In order to see them, we admit a pencil of solar light
chamber, through a very narrow opening, and at a
id tree or four yards, and we look at this opening
ps of flint glass, holding the edges parallel to
echink. We then observe a great number of
stripes, parallel to the edges of the prism, and
ly spaced. If we behold a spectrum with an
the streaks may amount to 600. We remark
which are more conspicuous than the others,
e been called the atria of Frainhafer, from the
topher who first noticed them. With lar
ks have fixed positions, which gives us the
with precision the index of each simple

be formed by artificial light, or by that of the
e positions are changed; while with electric
aks are replaced by light ones.

Prisma-When bodies are seen through
pris of their contours parallel to the edges
coloured with the tints of the
spectros

is explained by the unequal refrangibility
reflected by the bodies. If, firepl
y narrow slip of white paper pasted a black
whose edges shall be parallel to it, this
per bared with all the hoes of the spectrum, the
y deviated most towards the wit of the
periment the white light letel
deposed in its page hgh the
the most refrangible, har deviated most
elevated

led paper, instead of being very now, of a

all the middle part remains white; and
the edges of the primare ale min
ummit with ridlet mingel
bue with d

yellow. In order to explain this
e the piece of wide paper didel h
dipe. Each of these gives, is the fr
trim. Now the second spectrum Mag
rst, and the third a little lower then the
there results a websive perpation
which produce white, except ved
superposition is not complete re
ide and the red on the other s
farishes or with the means of lig
For this
fix upon black prom
ригро
Beholding it,
the yards, the light rela
its elements
which

be of the same substance, the refracting angle CFD of the second being smaller than the refracting angle BCF of the first, the two prisms will produce the same effect as one prism BAP; that is, the white light which passes through them will not only be bent, but decomposed. But if the first prism BCF, be made of crown glass, and the second of flint, we can destroy the dispersion, while preserving the refraction. The flint being more dispersive than the crown, and the dispersion produced by a prism diminishing with the angle of refraction in the prism, it follows that in suitably diminishing the angle of refraction CPD in the flint prism, with relation to the angle of refraction BCF in the crown prism, we can render the dispersive 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 BCF and CFD, 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 dispersive, and whose angles of refraction shall be suitably determined.

As to refraction, it is not corrected at the same time as dispersion, 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 substances unequally dispersive, the one ▲ in flint, is concave-convex-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 have seven glasses to obtain perfect achromatism; but in optical instruments two are sufficient, as they give the necessary curve to achromatise red rays and yellow rays.

Absorption of Light by Transparent Media. We know no substance which is perfectly transparent. Glass, water, air even, gradually tinge the light which passes through them, and with sufficient density these media will weaken it, so that it cannot act on the retina. We observe, for example, that a great number of stars, that are invisible on the plain let the sky be ever so clear, become visible when we ascend a high

mountain.

This gradual loss experienced by light in passing through transparent media is called absorption; and the cause of it is, the reflection which it undergoes upon the molecules of transparent bodies. If all the simple rays were equally transmissible through transparent substances, the latter would be colourless. Now this is never the case; for transparent bodies allow certain luminous rays to pass more easily than others. It is for this reason that, under great density, the air appears blue, and a plate of thick glass is green. Glass coloured red by protoxide of copper suffers only red rays to pass, and absorbs all the others, even when it is not thick. It is by the effect of absorption that the light of the sun is less intense, when that luminary is at the horizon, than when it is in the zenith, for then the density of the atmosphere is much more 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 telescopes. 3. Instruments designed to produce on a screen images, diminished or magnified, which may be useful in the art of design: these are the illuminated chamber, the dark chamber, the daguerreotype, the magic lantern, the fantasmagoria, the megascope, the solar microscope, and the gas microscope. The two first groups give only virtual images, and the last only real images, except the illuminated chamber.

The Simple Microscope.-We distinguish two sorts of microscopes, the simple and the compound. The simple microscope is sometimes formed by a single convergent lens, sometimes by several lenses superposed, which act as a single one. We have already seen that in the simple microscope, or magnifying glass, the object observed is placed between the lens and its principal focus, and that then the image is virtual, erect, and magnified.

Different dispositions are given to the simple microscope. Fig. 311 represents that which has been adopted by M

D

Fig. 311.

Raspail. A horizontal support, which can be elevated or depressed by means of a rack, and in which we see, at point D, a dark eye-hole, in the centre of which is placed a lens more or less convex. Beyond this is the holder B, which is fixed, and on which, between two glass plates c, is placed the object to be viewed. As it is necessary that the object should be strongly illuminated, we receive the light diffused through the atmosphere, on a concave glass reflector м, which is inclined in such a way that the reflected rays fall upon the object. In order to use this microscope, the eye must be placed very near the lens, which is elevated towards the object, or lowered, till we find the position in which the image is seen with most distinctness. With a simple microscope we can obtain a very distinct magnifying power to 120 times its diameter. The magnitude may be determined by calculation, or by experience, with the aid of the micrometer, which will be described by-and-by.

Compound Microscope.-The compound microscope, when reduced to its greatest degree of simplicity, is formed of two lenticular convergent glasses, the one named objective, because it is turned towards the object; the other, which is less convergent, is called ocular, because it is nearer the eye of the observer. Fig. 312 represents the progress of the luminous rays, and

[graphic]

the formation of the image in the compound microscope. An object, AB, being placed very near the principal focus of the objective, M, but a little beyond, with relation to this glass, an image, a b, real, inverted, and greatly amplified, is formed at the other side of the objective. Now the distance of the two glasses м and N is such that the place of the image a b is formed between the ocular, N, and its focus, It follows from this that to the eye placed at E, which

F.

beholds this image with the ocular, the last glass produces the effect of the simple microscope, and substitutes for the image a b a second image a'b', which is virtual, and magnified anew. This second image, erect in relation to the first image, is inverted in relation to the object. We may say, then, in regard to this last analysis, that the compound microscope is nothing but the simple microscope, applied not to the object, but to its image already magnified by the first lens. Magnitude-the Micrometer. The magnitude in every optical instrument, is the relation of the absolute size of the image to that of its object. Magnitude in the compound microscope is 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 experimentally by means of the micrometer-a small plate of glass, on which are traced with a diamond parallel lines, or do of a millimetre asunder. The micrometer is placed before the objective; then, instead of receiving directly on the eye the rays which emerge from the ocular o, we receive them on a plate of glass with parallel faces A (fig. 313), having an inclination of 45°, and we place the eye above so as to see the lines of the micrometer formed by reflection upon a scale divided into millimetres, which is traced on the screen E. Counting the number of divisions on the scale, which correspond to the number of lines of the image, we thence deduce the magnitude. For example, if the image occupy upon the scale 45 millimetres and comprehend 15 lines of the micrometer-supposing that the interval of these be of a millimetre-the absolute size of the object will be, and that of the image being 45 millimetres, the magnitude will be the quotient of 45 by 10% or 300. There is in this process, however, a source of error which results from the difficulty of placing the screen exactly

at the distance at which the eye sees the image of the micrometer.

The Compound Microscope of Amici.-We have hitherto described only the principle of the compound microscope. Our attention will now be directed to the principal accessories of this apparatus. Invented in 1620, it has from time to time received a number of improvements. The most important, however, date but thirty years back, and are due chiefly to M. Amici, in Italy, and M. Ch. Chevalier, in France.

Fig. 313.

E

Fig. 314 represents in its essential parts the microscope known as the Microscope of Amici, or the Microscope of Ch. Chevalier. In the ancient microscopes, the tube was always vertical, and the lenses were not achromatic. Amici was the first who adopted a disposition which allows the tube to be horizontal or vertical at pleasure, and it was Chevalier who, in 1823, first applied achromatic lenses to the microscope. Our diagram represents the microscope in the horizontal position, which is generally less fatiguing to the eye. But it may also be placed vertically. For this purpose, the elbow G is elevated, and the large tube A, which bears the ocular, rises to its place upon the objective E. Again, the microscope may have an inclined position. To give it this, we draw the bolt m, which secures the lower part of the apparatus, and we make the whole move on a hinge a, which binds the microscope to a cylindrical column serving for its support.

[merged small][graphic][subsumed][subsumed]

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 objectbearer. A glass concave reflector м, 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 its centre with a circular opening, which is perceived through the plates B, and which is designed for the passage of the light conveyed by the reflector.

The fig. 315 shows the position of the glasses, and the march

[blocks in formation]

of the rays. The objective is formed of one, two, or three achromatic lenses, as represented at K, the principal focal distances being from 8 to 10 millimetres. The ocular A H is formed of one simple lens, or two lenses, A and H, these being achromatic or not. It is easy to follow the march of the luminous rays. After being reflected on the mirror м, they meet towards the object, and thence they are directed towards the objective. After traversing that, they encounter a rectangular prism P, in crystal, on the hypothenuse of which they undergo a total reflection; then, taking the direction of the tube GA, the rays fall upon the lens H, and form, beyond, a real and magnified image of the object c. The last lens acts as a simple microscope, substituting for the first image a second image vertical and again magnified.

The intermediate image H has for its object to collect the too oblique rays which do not fall upon the ocular A. It enlarges the field of the microscope, while rendering the image smaller and more distinct. This glass serves also to correct the defect of achromatism, which the objective more or less presents. As to the aberration of sphericity, it is corrected by the screens m and n (fig. 315), which intercept the rays that tend to cross the lenses too near the edges. In order to extinguish all interior reflection, which could mar the distinctness of the images, the sides of the tube are blackened internally.

If the object be transparent, we illuminate it by means of a reflector placed under the object-bearer; if it be opaque, we use the lens L, supported by the object-bearer, which concentrates the light upon the object.

Finally, the apparatus has several spare oculars and objectives, by which the magnitude may be augmented or diminished. We can also diminish the magnitude by suppressing one or even two of the lenses of the objective.

The microscope has been the cause of the most interesting coveries in botany, in zoology, and in physiology. Aniwhose existence had been altogether unknown, have observed in vinegar, in water, in the paste of flour, in d fruits, and in certain kinds of cheese; while the circulan and the globules of the blood have become visible. The icroscope is also susceptible of numerous applications in the industrial arts. For example, it furnishes the means

LESSONS IN READING AND ELOCUTION. No. XIX.

EXERCISES ON EXPRESSIVE TONE-continued.

POETRY.

[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 respite 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 áim with Christianity; that is, to spiritualise our nature. Trùe, poetry has been made the instrument of vice, the pànder of bàd pássions; but when genius thùs stoops, it dìms its fires, and parts with much of its power; and even when Poetry is enslaved to licéntiousness and misanthropy, she cannot wholly forget her true vocation. Strains of pure feeling, 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 world, passages true to our moral náture, often escape in an immóral work, and show us how hard it is for a gifted spirit to divorce itself wholly from what is good.

Poetry has a natural alliance with our best affèctions. It delights in the beauty and sublimity of outward nature and of the soul. It indeed portrays with terrible energy the excèsses of the passions, but they are passions which show a mighty nature, which are full of power, which command awe, and excite a deep though shuddering sympathy. Its great tendency and púrpose, is, to carry the mind beyond and above the beaten, dusty, wèary walks of òrdinary life; to lift it into a purer element, and to breathe into it more profound and génerous emotion. It reveals to us the loveliness of nature, brings back the freshness of youthful feeling, revives the rèlish of simple pleasures, keeps unquenched the enthusiasm which warmed the spring-time of our being, refines youthful love, strengthens our interest in human nature, by vivid delineations of its tenderest and loftiest feelings, spreads our sympathies over all classes of society, knits us by new ties with univérsal being, and, through the brightness of its pròphetic visions, helps faith to lay hold on the future life.

We are aware that is it objected to poetry, that it gives wrong views, and excites fàlse expectations of life, peoples the mind with shadows and illusions, and builds up imagination on the ruins of wisdom. That there is a wisdom against which poetry wars,-the wisdom of the sènses, which makes physical comfort and gratification the supréme good, and wealth the chief interest of life,-we do not deny: nor do we deem it the least service which poetry renders to mankind, that it redeems them from the thraldom of this earthborn prùdence.

But, passing over this topic, we would observe, that the complaint against poetry as abounding in illùsion and decéption is, in the main, groundless. In many poems there is more of truth, than in many histories and philosophic theories. The fictions of génius are often the vehicles of the sublimest vèrities, and its flashes often open nèw regions of thought, and throw new light on the mysteries of our being. In poetry the letter is falsehood, but the spirit is often profoundest wisdom. And if truth thus dwells in the boldest fictions of the poet, much more may it be expected in his delinéations of life; for the présent life, which is the first stage of the immortal mind, abounds in the matérials of poetry, and it is the highest office of the bard to detect this divine element, among the grosser pleasures and labours of our earthly being. The present life is not wholly prosaic, precíse, tàme, and

A negative sentence, ending with a rising inflection, has the falling slide on its penultimate word or clause.

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