lens d, by which no such accident can happen. The lines o o, represent the incident rays of the sun, which are reflected on the condenser.

When the solar microscope is used, the room is darkened, the only light admitted being that which is thrown on the object by the condenser, and which, passing through the small lens, gives the magnified shadow e, of the small object a, placed in its focus, on the wall of the room, or on a screen. The tube containing the two lenses, is passed through the window of the room, the reflector remaining outside.

In the ordinary use of this instrument, the object itself is not seen, but only its shadow, on the screen, and it is not designed for the examination of opaque objects.

When the small lens of the solar microscope is of great magnifying power, it presents some of the most striking and curious of optical phenomena. The shadows of mites from cheese, or figs, appear nearly two feet in length, presenting an appearance exceedingly formidable and disgusting; and the insects from common vinegar appear eight or ten feet long, and in perpetual motion, resembling so many huge serpents.

Telescope. The telescope is an optical instrument, employed to view distant bodies, and in effect, to bring them nearer the eye, by increasing the apparent angles under which such objects are seen.

These instruments are of two kinds, namely, refracting, and reflecting telescopes. In the first kind, the image of the object is seen with the eye directed towards it; in the second kind, the image is seen by reflection from a mirror, while the back is towards the object, or by a double reflection, with the face towards the object.

The telescope is the most important of all optical instruments, since it unfolds the wonders of other worlds, and gives us the means of calculating the distances of the heavenly bodies, and of explaining their phenomena for astronomical and nautical purposes.

The principle of the telescope will be readily comprehended after what has been said concerning the compound microscope, for the two instruments differ chiefly in respect to the place of the object lens, that of the microscope having a short, while that of the telescope has a long focal distance.

Is the object, or only the shadow, seen by this instrument? What is a telescope? How many kinds of telescopes are mentioned? What is the difference between them? In what respect does the refracting telescope differ from the compound microscope?

Refracting Telescope. The most simple refracting telescope consists of a tube, containing two convex lenses, the one having a long, and the other a short focal distance. (The focal distance of a convex lens, it will be remembered, is nearly the centre of a sphere, of which it is a part.) These two lenses are placed in the tube, at a distance from each other equal to the sum of their two focal distances.

Thus, if the focus of the object glass a, fig. 169, be eight inches, and that of the eye glass b, two inches, then the dis. tance of the sums of the foci will be ten inches, and therefore the two lenses must be placed ten inches apart; and the same rule is observed, whatever may be the focal lengths of any two lenses.

Now to understand the effect of this arrangement, suppose the rays of light, cd, coming from a distant object, as a star, to fall on the object glass a, in parallel lines, and to be refract. ed by the lens to a focus at e, where the image of the star will be represented. This image is then magnified by the eye glass b, and thus in effect, is brought near the eye.

All that is effected by the telescope, therefore, is to form an image of a distant object, by means of the object lens, and then to assist the eye in viewing this image as nearly as possible by the eye lens.

ure,

It is, however, necessary here to state, that by the last fig. the principle only of the telescope, is intended to be explained, for in the common instrument, with only two glasses, the image appears to the eye inverted.

The reason of this will be seen by the next figure, where the direction of the rays of light will show the position of the image.

How is the most simple refracting telescope formed? Which is the object, and which the eye lens, in fig. 169? What is the rule by which the distance of the two glasses apart is found? How do the two glasses act, to bring an object near the eye?

a

Fig. 170.

d

Suppose a, fig. 170, to be a distant object, from which pen. cils of rays flow from every point towards the object lens b. The image of a, in consequence of the refraction of the rays by the object lens, is inverted at c, which is the focus of the eye glass d, and through which the image is then seen, still iverted.

The inversion of the object is of little consequence when the instrument is employed for astronomical purposes, for since the forms of the heavenly bodies are spherical, their positions, in this respect, do not affect their general appearance. But for terrestrial purposes, this is manifestly a great defect, and therefore those constructed for such purposes, as ship, or spy glasses, have two additional lenses, by means of which, the images are made to appear in the same position as the objects.

Fig. 171.

Such a telescope is represented at fig. 171, and consists of an object glass a, and three eye glasses, b, c, and d. The eye glasses are placed at equal distances from each other, so that the focus of one may meet that of the other, and thus the image formed by the object lens, will be transmitted through the other three lenses, to the eye. The rays coming from the object o, cross each other at the focus of the object lens, and thus form an inverted image at f. This image being also in

Explain fig. 170, and show how the object comes to be inverted by the two lenses. How is the inversion of the object corrected? Explain fig. 171, and show why the two additional lenses make the image of the object erect.

the focus of the first eye glass, b, the rays having passed through the glass become parallel, for, we have seen, in an. other place, that diverging rays are rendered parallel by refraction through a convex lens. The rays, therefore, pass parallel to the next lens, c, by which they are made to converge, and cross each other, and thus the image is inverted, and made to assume the original position of the object o. Lastly, this image, being in the focus of the eye glass d, is seen in the natural position, or in that of the object.

The apparent magnitude of the object is not changed by these two additional glasses, but depends, as in fig, 170, on the magnifying power of the eye and object lenses; the two glasses being added merely for the purpose of making the image appear erect.

It is found that an eye glass of very high magnifying pow er cannot be employed in the refracting telescope, because it disperses the rays of light, so that the image becomes indis. tinct. Many experiments were formerly made with a view to obviate this difficulty, and among these it was found that increasing the focal distance of the object lens, was the most efficacious. But this was attended with great inconvenience, and expense, on account of the length of tube which this mode required. These experiments were, however, discontinued, and the refracting telescope itself, chiefly laid aside for astronomical purposes, in consequence of the discovery of the reflecting telescope.

Reflecting Telescope.-The common reflecting telescope consists of a large tube, containing two concave reflecting mirors, of different sizes, and two eye glasses. The object is first reflected from the large mirror to the small one, and from the small one, through the two eye glasses, where it is then

seen.

In comparing the advantages of the two instruments, it need only be stated, that the refracting telescope, with a focal length of a thousand feet, if it could be used, would not magnify distinctly more than a thousand times, while a reflecting telescope, only eight or nine feet long, will magnify with distinctness twelve hundred times.

Does the addition of these two lenses make any difference with the apparent magnitude of the object? Why cannot a highly magnifying eye glass be used in the telescope? What is the most efficacious means of increasing the power of the refracting telescope? How many lenses and mirrors form the reflecting telescope? What are the advantages of he reflect ing over the refracting telescope?

Fig. 172.

b

The principle, and construction of the reflecting telescope will be understood by fig. 172. Suppose the object o to be at such a distance, that the rays of light from it pass in parallel lines, p, p, to the great reflector, r, r, This reflector being concave, the rays are converged by reflection, and cross each other at a, by which the image is inverted. The rays then pass to the small mirror, b, which being also concave, they are thrown back in nearly parallel lines, and having passed the aperture in the centre of the great mirror, fall on the plano-convex lens e. By this lens they are refracted to a focus, and cross each other between e, and d, and thus the image is again inverted, and brought to its original position, or in the position of the object. The rays then, passing the second eye glass, form the image of the object on the retina.

The large mirror in this instrument is fixed, but the small one moves backwards and forwards, by means of a screw, so as to adjust the image to the eyes of different persons. Both mirrors are made of a composition, consisting of several metals melted together.

One great advantage which the reflecting telescope possess. es over the refracting, appears to be, that it admits of an eye glass of shorter focal distance, and consequently, of greater magnifying power. The convex object glass of the refracting instrument, does not form a perfect image of the object, since some of the rays are dispersed, and others coloured by refraction. This difficulty does not occur in the reflected image from the metallic mirror of the reflecting telescope, and consequently it may be distinctly seen, when more highly magnified. The instrument just described is called "Gregory's teles

Explain fig. 172, and show the course of the rays from the object to the eye. Why is the small mirror in this instrument made to move by means of a screw? What is the advantage of the reflecting telescope in respect to the eye glass? Why is the telescope with two reflectors called Gregory's telescope?