the axis of the tube, and the image i, being thus thrown to one side of the tube, is there viewed by the eye-piece.

el

S

We shall now proceed to explain the best methods of adjusting and testing telescopes, as given by Pearson in his valuable work on Practical Astronomy.

Methods of Adjusting and Testing Refracting Telescopes.-Let us suppose that we have a refracting telescope of 3 feet focal length, and 34 inches aperture. Then, to test the object-glass, lay the tube of the telescope in a horizontal position upon some fixed support about the height of the eye, and place a printed card vertically, but for a celestial eye-piece in an inverted position, against some wall or pillar at thirty or forty yards' distance, so as to be exposed to a clear sky; then, when the telescope is directed to this object, and adjusted by the sliding tube for distinct vision, the letters on the card should appear clearly and sharply defined, without any colouration or mistiness; and, if very small points appear well defined, the object-glass may be deemed a pretty good one for viewing terrestrial objects. If the glass be intended for astronomical observations, fix at the same distance a black board, or one-half of a sheet of black paper, and a circular disc of white paper, about a quarter of an inch or less in diameter, upon the center of the black ground; then having directed the telescope to this object, and adjusted for distinct vision, mark with a black-lead pencil the sliding eye-tube, at the end of the main tube, so that this position can always be known; and if this sliding tube be gradually drawn out, or pushed in, while the eye beholds the disc, it will gradually enlarge and lose its colour, till its edges cease to be well defined. Now, if the enlarged misty circle is observed to be concentric with the disc itself, the object-glass is properly centered, as it has reference to the tube; but, if the misty circle goes to one side of the disc, the cell of the object-glass is not at right angles to the tube, and must have its screws removed, and its holes elongated by a rat-tailed file, small enough to enter the holes. When this has been done, replace the cell, and examine the disc a second time, and a slight stroke on the edge of the cell by a wooden mallet will show, by the alteration made in the position of the misty portion of the disc, how the adjustment is to be effected, which is known to be right when a motion in the sliding tube will make the disc enlarge in a circle concentric with the disc itself. When, then, the disc will enlarge so as to make a ring of diluted white light round its circumference, as the sliding tube holding the eye-piece is pushed in, or drawn out, the cell may be finally fixed by the screws passing through its elongated holes. When the object-glass is thus adjusted, we can proceed to ascertain whether the curves of the respective lenses composing the object-glass are well formed and suitable for each other.

If a small motion of the sliding tube of about one-tenth of an inch from the point of distinct vision, in a 3-feet telescope, will dilute the light of the disc and render the appearance confused, the figure of the object-glass is good; particularly if the same effect will take place at equal distances from the point of good vision, when the tube is alternately drawn out and pushed in, Such an object-glass is said to be aplanatic. A telescope that will admit of much motion in the sliding tube without affecting sensibly the distinctness of vision will not define an object well at any point of adjustment, and must be considered as having an imperfect object-glass in which the spherical aberration is not duly corrected. The achromatism of the object-glass is to be judged of by the absence of colouration round the enlarged disc. When an object-glass is free from imperfection both in respect of its aplanatism and achromatism, it may be considered a good glass for all terrestrial purposes.

How far an object-glass is good for astronomical observations can only be determined by actual observation of a heavenly body. When a good telescope is directed to the Moon, or to Jupiter, the achromatism may be judged of by alternately pushing in, and drawing out, the eye-piece, from the place of distinct vision; in the former case a ring of purple will be formed round the edge; and, in the latter, a ring of light green, which is the central colour of the prismatic spectrum; for these appearances show that the extreme colours, red and violet, are corrected. Again, if one part of a lens employed have a different refractive power from another part of it, that is, if the glass, particularly flint glass, be more dense in one part than another, a star of the first, or even of the second magnitude will point out the natural defect by the exhibition of an irradiation, or what opticians call a wing at one side, which no perfection of figure or adjustment will banish; and, the greater the aperture, the more liable is the evil to happen.

Another method of determining both the figure and quality of the objectglass is by first covering its center by a circular piece of paper, as much as one-half of its diameter, and adjusting for distinct vision of a given object, which may be the disc above mentioned, when the central rays are intercepted, and then trying if the focal length remains unaltered, when the paper is taken away, and an aperture of the same size applied, so that the extreme rays may in their turn be cut off. If the vision remains equally distinct in both cases, without any new adjustment for focal distance, the figure is good, and the spherical aberration cured; and it may be seen, by viewing a star of the first magnitude successively in both cases, whether the irradiation is produced more by the extreme, or by the central parts of the glass; or, in case one-half of the glass be faulty and the other good, a semicircular aperture, by being turned gradually round in trial, will detect what semicircle contains the defective portion of the glass; and, if such portion should be covered, the only inconvenience that would ensue would be the loss of so much light as is thus excluded.

The smaller a large star appears in any telescope, the better is the figure of the object-glass; but, if the image of the star be free from wings, the size of its disc is not an objection in practical observations, as it may be bisected by the small line by which the measure is to be taken. When, however, an object-glass produces radiations in a large star, it is unfit for the nicer purposes of astronomy. In testing a telescope, if a glass globe be placed at 40 yards distance when the sun is shining, the speck of light reflected from this globe forms a good substitute for a large star, as an object to be viewed.

Whenever an object-glass is under examination, it will be proper to have the object examined by it in the center of the field of view; and, when an object-glass is tested for astronomical purposes by the methods described

above, it is necessary to employ a good negative eye-piece, which generally gives a better field of view than the positive.

If any fringes of red or yellow are observed on the edges of a white disc placed on a black ground, when the telescope is adjusted for distinct vision, and the disc carried too near the edges of the field, this species of colouration indicates that the eye-piece is not sufficiently free from spherical aberrations; and, if the curves of the lenses are suitable for each other, the cure is effected by an alteration in the distance between them, which must be finally adjusted by trial with a good object-glass.

Methods of Adjusting and Testing Reflecting Telescopes.--To adjust the specula of a Cassegrainian or Gregorian instrument procure a Ramsden's eye-piece, which will render an object visible in the compound focus of the two lenses of which it is composed; then hold this eye-piece in front of the Huyghenian eye-piece of the telescope, and, by varying the distance, find the position in which the image of the large speculum is seen, well defined through both eye-pieces, and, if the image of the small speculum is seen precisely on the center of the large one, the metals may be considered as rightly placed; but, if not, the proper screws must be used in succession, till the required position is determined. When the face of the large metal stands at right angles to the length of the tube, the adjustment may generally be finished without disturbing it; and, when the bed that receives it has once been properly finished, it will be advisable not to alter it, unless some accident should render such alteration indispensable.

To try whether the figures of the metals are adapted for each other.-Let the instrument be directed to some luminous point, as a white disc on a black ground, or, what is better, to a star: then having adjusted for distinct vision, firstly observe if the disc or star is well defined, and free from irradiations; secondly, carrying the small speculum short distances beyond, and short of, the place for distinct vision, examine if the disc or star enlarges alike in similar changes of position: if the result be satisfactory, the metals may be considered as well placed, and well adapted for each other.

To try whether the large speculum partake of the parabolic form, let the aperture be partially covered, first at the central part, and then round the circumference by tin, pasteboard, or stiff paper; and if on trial the same adjustment for distinct vision be good in both these cases, and also when the speculum is all exposed, the figure may be considered good. If these effects be not produced, the instrument will be incompetent to perform several of the nicer observations in astronomy. When a mistiness appears in the field, it is a proof that the aberrations are not corrected, and that the figure of at least one of the specula is not perfect.

If a telescope is not good with its full aperture, its effect may be greatly improved, by putting a cover on the mouth, with a circular aperture, of about one-half the diameter that the tube has, in such a way that the diminished aperture may fall entirely at one side of the opening of the tube.

THE SOLAR MICROSCOPE.

In this instrument the object itself is not viewed through a combination of lenses, as in the microscopes already described (pp. 76-81), but a magnified image of the object is formed by a combination of lenses, and received upon a screen The term solar is applied to the instrument, because the light of the sun, concentrated by a lens, is made use of to illuminate

the object to be observed, and the construction is in all other respects identical with the common magic lantern, and the oxy-hydrogen microscope. In the case of the microscope, how ever, whether illuminated by the sun or the brilliant oxyhydrogen light, great regard must be had to the forms of the lenses and the perfection of the setting; while a comparatively very rough instrument forms a very amusing toy as a magic lantern, exhibiting grotesque figures and scenes, which are painted in transparent colours upon glass slides.

The arrangement of the apparatus will be understood from the annexed diagram; r is a reflector for turning the sun's

m

rays in a direction parallel to the axis of the instrument: c is the lens for concentrating these rays upon the object placed at o, a little further from the first lens, p, of the magnifier, than the focal length of this magnifier, which is one-fourth the focal length of p; then we have p and m, the two lenses forming the magnifier, which are of equal focal length, and separated by an interval equal to two-thirds of the common focal length, as in Ramsden's positive eye-piece: lastly comes the diaphragm, d, placed at a distance from m, the second lens of the magnifier, equal to the focal length of this magnifier, which is one-fourth the focal length of m or p.

The best forms of the two lenses are, for the first, a planoconvex, and, for the second, a convex meniscus, the radii of whose surfaces are as 1 to 15; and the advantage aimed at in this construction is to render the image flat, and consequently capable of coinciding with the plane screen upon which it is to be received. A similar purpose is the object of the construction of Ramsden's eye-piece, viz., to obtain, as it is there called, a flat field.

The object being placed a little further from p than the focal length of the magnifier, the pencils of rays from each point of the object, after passing through the two lenses, become slightly convergent, and, at a distance from the diaphragm depending upon the distance of the object from the lens p, the magnified image is formed inverted with respect to the object

THE CAMERA OBSCURA.

This instrument consists of a plane reflector, upon which

pencils of light from the various points of a landscape are re ceived and reflected, so as to pass first through a diaphragm, and then through a plano-convex lens, after which the rays of the pencils become convergent, and form an image upon a screen in a darkened chamber placed to receive it. The diaphragm and lens are placed in a tube, which is passed through a hole in the chamber just large enough to receive it, so that no extraneous light may be admitted. The distance of the lens from the diaphragm is determined upon the condition that the image shall be distinct. The form of the screen also, that the image may be distinct, is a paraboloid of revolution, or figure formed by the revolution round its axis of a parabola, whose radius of curvature at the vertex is μ ƒ, μ being the refracting power of the medium of which the lens is formed, and f the focal length of the lens. A curved surface of this form is, therefore, made of plaster of Paris, and placed at a distance from the lens rather greater than the focal length, the exact distance depending upon the nearness or remoteness of the landscape to be depicted, and being easily found by trial. If the camera be set up in the neighbourhood of a well-frequented thoroughfare, we have then an agreeable succession of distinct and vividly-coloured pictures, differing from finely-executed paintings only by exhibiting the actual motions of the objects viewed, men walking, horses trotting, soldiers marching, banners streaming, and foliage shaking in the breeze.

THE CAMERA LUCIDA.

This ingenious instrument, the invention of Dr. Wollaston, consists of a quadrilateral prism, of which A B C D represents a section made by a plane at right angles

to each of its edges, mounted upon an axle parallel to its edges. This axle is attached to the end of a rod sliding in a tube, which has at the other end a clamp for fixing it to the edge of a table, so that the distance of the prism from the table can be shortened or lengthened at pleasure. A B is equal

T

P

B

to B C, and A D to D C, and the angles of the prism are a right angle at B, an angle of 135° at D, and angles each 67° 30′ at A and c. Over the face B A, and projecting beyond ▲, is a plate of metal having in it a narrow longitudinal aperture, which is just bisected by the edge A of the prism.