place of the object lens, that of the microscope having a short, while that of the telescope has a long, focal distance.

Refracting Telescope. The most simple refracting telesscope 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 double 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.

Fig. 169.

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 distance 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, c d, coming from a distant object, a« a star, to fall on the object glass a, in parallel lines, and to be refracted 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.

It is, however, necessary here to state, that by the last figure, 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?

Fig. 170. b

Suppose a, fig. 170, to be a distant object, from which pencils of rays flow from every point toward 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 inverted.

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. These are called double telescopes.

Fig. 171.

Such a telescope s 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 the

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.

focus of the first eye glass, b, the rays having passed through this glass become parallel, for, we have seen, in another 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 power cannot be employed in the refracting telescope, because it disperses the rays of light, so that the image becomes indistinct. 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 mirrors, 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 the reflecting over the refracting telescope?

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 orignal 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 possesses 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 colored 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 telescope," because some parts of the arrangement were invented by Dr. Gregory.

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?

In the telescope made by Dr. Herschel, the object is reflected by a mirror, as in that of Dr. Gregory. But the second. or small reflector, is not employed, the image being seen through a convex lens, placed so as to magnify the image of the large mirror, so that the observer stands with his back towards the object.

The magnifying power of this instrument is the same as that of Dr. Gregory's, but the image appears brighter, because there is no second reflection; for every reflection renders the image fainter, since no mirror is so perfect as to throw back all the rays which fall upon its surface.

In Dr. Herschel's grand telescope, the largest ever constructed, the reflector was 48 inches in diameter, and had a focal distance of 40 feet. This reflector was three and a halt inches thick, and weighed 2000 pounds. Now, since the focus of a concave mirror is at the distance of one half the semidiameter of the sphere, of which it is a section, Dr. Herschel's reflector having a focal distance of 40 feet, formed a part of a sphere of 160 feet in diameter.

This great instrument was begun in 1785, and finished four years afterwards. The frame by which this wonder to all astronomers was supported, having decayed, it was taken down in 1822, and another of 20 feet focus, with a reflector of 18 inches in diameter, erected in its place, by Herschel's son.

The largest Herschel's telescope now in existence is that of Greenwich observatory, in England. This has a concave reflector of 15 inches in diameter, with a focal length of 25 feet, and was erected in 1820.

Camera Obscura. Camera obscura strictly signifies a darkened chamber, because the room must be darkened, in order to observe its effects.

To witness the phenomena of this instrument, let a room be closed in every direction, so as to exclude the light. Then from an aperture, say of an inch in diameter, admit a single beam of light, and the images of external things, such as trees, and houses, and persons walking the streets, will be seen inverted on the wall opposite to where the light is admitted, or on a screen of white paper, placed before the aperture.

How does this instrument differ from Dr. Herschel's telescope? What was the focal distance and diameter of the mirror in Dr. Herschel's great telescope? Where is the largest Herschel's telescope now in existence? What is the diameter and focal distance of the reflector of this telescope Describe the phenomena of the camera obscura.