Page images
PDF
EPUB

By substituting for the convex eye-lens of the astronomical telescope a concave eye-lens of the same focal length, a simple telescope is formed with only two lenses, which shows objects erect. This is called the Galilean

telescope, and is the construction used for opera glasses. When arranged for distinct vision of a

distant object, the object-glass and eye-lens are separated. by a distance equal to their focal lengths. The pencils of light proceeding from the object, after refraction at the object-glass o, tend to form an image of the object in the common focus of the two lenses; but, being intercepted by the concave eye-lens e, their rays are rendered parallel, and, consequently, adapted to produce distinct vision to an eye placed behind this lens.

The magnifying power, as in the astronomical telescope, is represented by the ratio of the focal length of the object-glass to that of the eye-lens.

Reflecting Telescopes.-Since the discovery of the methods. of forming achromatic and aplanatic object-glasses, the magnitude and available magnifying powers of refracting telescopes are theoretically unlimited; but the difficulty of procuring flint glass of even texture and free from flaws, in pieces of any considerable magnitude, has hitherto practically placed a limit upon the magnitude and available power of refracting telescopes. By the substitution, however, of reflectors, which are always achromatic, for the object-glasses, telescopes of colossal magnitude have been most successfully constructed. Of reflecting telescopes there are four kinds-the Newtonian, the Gregorian, the Cassegrainian, and the Herschelian. The Newtonian telescope consists of a concave object-speculum, s, a plane reflector m, making an angle of 45° with the axis of the

[ocr errors]

m

telescope, placed between the object-speculum and its focus, and an eye-piece. The pencils of light proceeding from a distant object tend to form an image after reflection at the object-speculum, but are bent by the plane reflector, so that the image is formed at i, on the axis of the eye-piece, and in the focus of the eye-lens.

The Gregorian telescope consists of a concave object-speculuin, s, a small concave speculum, r, whose focal length is

short compared

with that of the

object - speculum, and an eye-piece. The

r

small speculum is placed so that its focus is near the focus of the object-speculum, but a little further from this speculum. The pencils of light proceeding from a distant object, after reflection at the object-speculum, form an inverted image, h, of the object at the focus of this speculum, and after reflection again at the small speculum form a second image, i, inverted with respect to the former, and, consequently, erect with respect to the object.

This telescope has, for terrestrial purposes, the advantage over the Newtonian telescope, of showing objects erect, but yields to it both in the brightness and perfection of the image, because the second mirror increases the spherical aberration produced by the first, and it is extremely difficult to give the mirrors the proper curvature to remedy this evil.

The Cassegrainian telescope consists of two specula and an eye-piece, like the Gregorian, but the second speculum is convex instead of concave, and is placed between the objectspeculum and its principal focus, at a distance from this focus somewhat less than its own focal length. The pencils of light proceeding from a distant object, after reflection at the objectspeculum, tend to form an inverted image of the object, but are intercepted, before doing so, by the convex speculum, and made to form the image still inverted, in the focus of the eyelens. Objects, therefore, are still inverted; but the spherical aberration of the convex speculum being opposite to that of the concave object-speculum, the whole spherical aberration is diminished. This telescope is also shorter than the Gregorian. It is, however, inferior to the Newtonian telescope for celestial observations, and not well adapted for terrestrial purposes on account of the inversion of the object.

When light is reflected at a mirror or speculum, there will always be a waste and dispersion; and in consequence of the two reflections, and also of the light intercepted by the plane mirror, or second speculum, the loss of light in all the reflecting telescopes hitherto described is considerable. Sir W Herschel, by a very simple contrivance, obtained what is called the front view; but this construction is only applicable to in struments of very large dimensions. In the Herschelian telescope the axis of the object-speculum, s, is slightly inclined to

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.

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 it 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 mus 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

« PreviousContinue »