ON PHYSICS, OR NATURAL PHILOSOPHY. No. L. (Continued from page 350.) OPTICS. OPTICAL INSTRUMENTS. The Telescope.—The telescope (from the Greek words, rne, at a distance, and σKоTEW, I see) is an optical instrument by which distant objects are brought within the range of distinct vision. The astronomical or celestial telescope is composed of a convergent object-glass and a convergent eyepiece. The object м, fig. 316, gives an inverted image of the Fig. 316. M body which we look at a b, situated between the eye-piece N, and its principal focus; and this eye-piece, which has the effect of a magnifying lens, gives a virtual image, a' b', erect and greatly magnified, of the image a b. The astronomical telescope has, as we see, considerable analogy with the microscope; but these instruments present this difference, that, in the the focus of the object-glass м, and being supposed to coincide approximately with the focus of the eye-piece N, whence we conclude that the eye-piece is more considerable as the objectglass is less and the eye-piece more convergent. In a good glass the magnifying power does not exceed 1000 to 1200. Fig. 317 represents a telescope mounted on a stand, and above it is a small telescope called a searcher. Glasses of great magnifying power, having but a narrow field, are not convenient for seeking a star. For this reason we examine the heavens first with the searcher, which commands a large field; and having discovered the object, we then examine it with the telescope. Terrestrial Telescope.-The terrestrial telescope differs from the celestial only in this, that the images are rectified. This rectification is obtained by means of two convergent glasses, P and Q (fig. 318), placed between the object-glass м, and the eye-piece R. The object being supposed at A B, at a much greater distance than can be represented in the diagram, its image is formed, inverted and very small, at ba, on the other side of the object-glass. Now the second lens P is at such a distance that its principal focus coincides with the image a b; whence it follows that the luminous rays that pass through b, for example, take, after having traversed the lens P, a direction parallel to the secondary axis bo. In like manner the rays which pass through a take a direction parallel to the axis a 0. Fig. 317. microscope, the object being very near the object-glass, the | image is formed far beyond the principal focus, and is greatly amplified, magnitude being produced both by the object-glass and the eye-piece, while in the celestial telescope the body which we behold being very distant, the incident rays are parallel, and the image is formed at the principal focus of the object-glass, much smaller than the object. There cannot, then, be magnifying power except by the eye-piece, and for that reason this glass should be very convergent. After crossing at H, these different rays traverse a third lens, Q, The pencil whose principal focus coincides with the point H. of rays, Bb H, then converges at b' on a secondary axis, o'b', parallel to its direction. The pencil a a H, proceeding in like manner to converge at a', a rectified image of the object A B is produced at a' b'. It is this image which we behold with a convergent eye-piece, R, so placed that it acts as a magnifying lens, that is to say, its distance from the image ab is less than the principal' focal distance; whence it results that it gives at a" b" a virtual, erect, and magnified image of a' b', 1 THE 2. Porta, at the end of the Beory', Tlescope.-Fig. 320 represents the telescope of Crory, mounted on a stand, around which it may be turn, ani mc.e or less inclined. Fig. 321 gives a longitu linal U 44 and the second mirror is plane, with an elliptical contour, and inclined at an angle of 45 degrees to the axis of the telescope. This plane mirror is situated between the reflector м and its focus, a little nearer to the focus than to the eye-piece o, placed in the side of the telescope. The result of this arrangement is that, after being reflected on the mirror м, and then on the mirror N, the rays form at ab an inverted and very small image of the heavenly body, between the eye-piece o and its principal focus. This glass produces, then, the effect of a magnifying glass, and gives at a' b' an enlarged, but inverted, image of the luminary. 3. Herschel's Telescope.-The telescope of Herschel, attributed also to Lemaire, is formed of only one concave reflector, (fig. 323), and of an eye-piece, o. The reflector is inclined to Fig. 323. M the axis in such a way, that the image of the star we observe is formed at ab on the side of the telescope, near the eye-piece o, which then gives the magnified image a'b'. In this telescopethe rays experiencing but a single reflection-the loss of light is less than in the two preceding ones, and the image is clearer. With regard to the magnifying power, it is, as in the others, the ratio of the principal focal distance of the mirror to that of the eye-piece. Camera Obscura.-The camera obscura, or dark chamber, is a room which excludes the light in every part, with the exception of a small aperture, by which the luminous rays enter. Then all external objects whose rays can reach the opening are painted on the wall opposite, with reduced dimensions, and with their natural colours; but the images are inverted. Porta, a Neapolitan philosopher, discovered, in 1560, the phenomenon produced by a luminous pencil which penetrates through a hole into a dark chamber. Shortly after, the same philosopher observed that if in the opening you fix a bi-convex lens, and place at its focus a white screen, the image which is produced gains considerably in brilliancy, in distinctness, in colour, and is admirably true. These images are bright in proportion to the size of the lens, and their dimensions augment with the focal distance. In order to render the camera obscura useful in the art of design, various forms are given to it, so as to render it portable, and to rectify the images with facility. Fig. 324 represents the sliding dark chamber. It consists of a rectangular wooden box, into which the luminous rays R penetrate through the lens B, and form an image on the opposite side, o, which ought to be distant from the lens B by a length equal to its focal distance. Now the rays encountering a glass mirror м, with an inclination of 45°, change their direction, and the image is formed on a screen of polished glass N. By placing upon this screen a sheet of paper, we can take the outline of the image with fidelity. The screen a serves to intercept the light which would illumine the image, and prevent the view. The box consists of two parts, which slide by means of a groove into one another, so that the anterior part being drawn out more or less, the image is formed, after reflection, exactly on the screen N, whatever be the distance of the object which we wish to sketch. Fig. 325 represents another kind of camera obscura, the dark chamber with a prism. In a brass case, A, there is a triangular prism, r (fig. 326), which occupies the place at once of a convergent lens and a mirror; for that purpose, one of its surfaces being plane, the others have a curvature of such a nature that, by their combined refractions, at the entrance and exit of the rays, they produce the effect of a convergent meniscus, c. From this it follows that the rays emitted by the object A B, after having entered the prism, and experienced total reflection on the surface cd, form at ab a real image of A B. This being understood, and the tablet B corresponding to the focus of the prism contained in the case A, the images of external objects are formed on a piece of paper placed upon this tablet. e -eive L T T The lenses P and a, which serve fixed in a brass tube, at a distance to the sum of their principal 1 object-glass M, it is moveable in approach to, or recede from, the 1 the image ab is always formed at ever be the distance of the object tance of the lens R may also b shall be made within the range The terrestrial telescope m tial, but for this it is neces which must have more magn the former. Astronomers, } two glasses, because it abso:! power is the same as in th the rectifying glasses Pard Galileo's Telescope.-The simple of all, for it is com vergent object-glass M, a and gives a rectified in represented by the verted, real, and piece R, the rays e in quitting, respe correspond to ti. that the rays, p T light, and in which the dark and light tints occupied the same place as the object. In 1829, Niepce communicated his process to Daguerre, who was already known by the invention of the diorama, and hil himself been engaged for several years in similar researches; but it was not till after the labour of ten years that he made, in 1839, the beautiful discovery which has made so much noise in the world. Niepce did not live to share the glory, having died three years before. The process of Daguerre consists of five principal operations. 1 The polishing of the thin brass-plate, coated with silver, on which the image is to be formed. 2. Depositing upon this plate the sensible coating, that is, the substance susceptible of impression from the light. 3. The exposure of the plate in a dark chamber to the action of the light. 4. The exposure of the plate to the mercurial vapours which cause the image to appear. 5. The fixation of the image. When the preliminary processes are gone through, the plate is placed in a little portable dark chamber made of wood, which is represented in fig. 329, and which commonly receives Fig. 329. to 75°, by means of a small lamp supplied with alcohol, the mercurial vapours are copiously deposited in the form of small imperceptible drops on those parts which have been strongly illumined, and in the course of a few minutes there is formed an amalgamation of silver and mercury, which gives the white portions of the picture, while the other parts remain dark. The image is then visible, and remains when exposed to the sun. However, the plate is still covered, especially in the shade, with a coat of iodine of silver, which gives to the picture a tinge of yellow, mingled with red or violet. This tinge can be made to vanish by washing the plate in a solution of sulphate of soda. But the image will not resist the slightest friction, which proves that the silver and the mercury have not amalgamated. There remains, therefore, another operation by which this defect is corrected. The plate is washed in a weak solution of chloride of gold and sulphate of soda. By this operation the silver is dissolved, while the gold is combined with the mercury and the silver of the plate; the amalgamation of the mercury and silver, which forms the white of the picture, then increases the solidity and brightness, by combining with the gold, whence results a remarkable increase of intensity in the lights of the picture. It is to M. Fizeau that we owe the employment of chloride of gold, which is the principal improvement that has been made upon the invention of Daguerre. Fig. 330 represents a section of the object-glasses, that is, of D B the apparatus which serves to concentrate the light upon the plate, and to produce the image. It consisted at first of a single bi-convex achromatic lens, but very soon object-glasses consisting of two achromatic lenses were adopted. They the name of daguerreotype. This is composed of a fixed part focal distance, and permit an easy adjustment of the focus, operate more quickly than single object-glasses, have a smaller c, and a moveable part B, and is a real sliding camera obscura.which is accomplished by causing the lens в to approach or In a brass tube A, is the object-glass. It is a convergent achro- recede, it being turned towards the object by means of a rack matic lens, which advances or recedes, by means of a screw and a tooth D. which can be turned with the hand and the button D. The side opposite the object-glass is formed of a polished glass screen fixed in the frame F, which is elevated at will. Matters being thus arranged, in order to obtain a portrait, the person is made to sit four or five yards in front of the object-glass, then we draw the moveable case B, till the image, which is produced inverted on the plate of glass, has appeared with distinctness, which happens when the plate is apparently at the focus. It is put at the focus by advancing or withdrawing the object-glass by means of the button D. For portraits, the focus is adapted to the eyes of the person sitting, this part of the face being the most central. The focus being found, without displacing the dark chamber, we elevate the frame E and the glass screen, and put in its place the case which contains the plate with iodine. Finally, withdrawing the screen which masks the silvered surface, the image which is formed on the glass is formed also on the plate. It is then that the light produces its mysterious effect, and paints upon the plate an invisible image. The time during which the plate should be exposed to the light varies with the object-glass, with the preparation of the sensible coating, and with the intensity of the light-it may be from 8 to 50 seconds. If the exposure be too much prolonged, the picture will be white: it will be black if the exposure is too short. When the time has come to arrest the action of the light, which long practice alone can enable us to determine, we lower the screen, and withdraw the case, in which the plate rests in total darkness. If we observe the plate at this moment, we do not yet perceive any trace of the image. In order to render it visible, it must be exposed to the action of the vapour of mercury, by placing it, at an inclination of 45°, in the upper part of a wooden box, prepared for this purpose, the bottom of which, consisting of sheet iron, contains a cavity full of mercury. This being brought to a temperature of 60° FEMALE EDUCATION.-No. II. BY SILVERPEN. HABITS.-So much of a young woman's refinement and improvement depend on daily habits, as to make it impossible to expect mental or moral progress where these are coarse, low, and habitually vulgar. In this respect, I am well aware how many evils the young women of the operative classes have to contend against, in respect to poor and often low houses, worse companions and neighbours, the want of privacy and good example; but I have seen so much effected by them in individual instances, so much which is genteel and womanly, as to assure me that in the majority of cases the duties of moral refinement and self-respect need but be pointed out to make them objects of solicitous acquirement. I have a grand belief in the innate modesty and purity of my sex; as strong an assurance that it is through the process of refinement women will yet largely increase the moral bounds of social action; and I do think that any young woman, let surrounding evils be what they may, can correct the influence of half of them if she will but reason with herself and say, "I have an individual mind and body distinct from that of others; I have youth and health and a fresh mind; let me then strive to rightly use and make the best of these, in spite of my low companions, my coarse mother, my ill-conducted home. What right have I to be bowed down by these to an equal degradation? No! let me exercise with discretion these sweet gifts of body and mind, let me show myself a young woman of the onward age, and not the mere slave of the habits and manners of those arround me; but superior as far |