501. The focal distance of a plano-convex lens is equal to the diameter of the sphere formed by the convex surface produced.

It must be borne in mind that light is refracted both when it enters and when it leaves a double-convex lens, and in both instances in the same direction; and, so far as the distance of the focus is concerned, to the same extent. But when the lens is convex only on one side, half its refracting power is gone, so that the rays are not so soon refracted to a focus. In this case, the focal distance is equal to the diameter of the sphere

B

PLANO-CONCAVE-FOCAL DISTANCE.

formed by extending the convex surface of the lens; while with the double-convex lens, the focal distance is only equal to the radius of such sphere. In the cut, the parallel rays A are refracted to a focus at B, by the plano-concave lens C; and the distance CB is the diameter of the circle D, formed by the convex surface of the lens C produced.

502. A doubleconcave lens disperses parallel rays, as if they diverged from the center of a circle formed by the convex surface produced.

RAYS DISPERSED BY REFRACTION.

A

In this cut, the parallel rays A are dispersed by the double-concave lens B, as shown at C; and their direction, as thus refracted, is the same as if they proceeded from the point D, which is the center of a circle formed by the concave surface of the lens produced.

500. How focal distance governed? (Diagram.)
501. What is the focal distance of a plano-convex lens

(Diagram.)

502. Effect of double-convex lens? Amount of divergency of rays? (Diagram.)

503. Common spectacles, opera-glasses, burning-glasses, and refracting telescopes are made by converging light to a focus, by the use of double-convex lenses.

BURNING-GLASS.

The ordinary burning-glass, which may be bought for a few shillings, is a double-convex disk of glass two or three inches in diameter, inclosed in a slight metallic frame with a handle on one side. Old tobacco-smokers sometimes carry them in their pockets, to light their pipes with when the sun shines. In other instances, they have been so placed as to fire a cannon in clear weather, by igniting the priming at 12 o'clock.

The adjoining cut represents a large burnIng-glass converging the rays of the sun to a focus, and setting combustible substances on fire. Such glasses have been made powerful enough to melt the most refractory sub

stances, as platinum, agate, &c. "A lens three feet in diameter," says Professor Gray, "has been known to melt carnelian in 75 seconds, and a piece of white agate in 80 seconds."

REFLECTION OF LIGHT.

504. We have now shown how light may be turned out of its course, and analyzed, dispersed, or converged to a point by refraction. Let us now consider how it may be converged to a focus by reflection.

505. When light falls upon a highly polished surface, especially of metals, it is reflected or thrown off in a new direction, and the angles of contact and departure are always equal.

Let A B represent the polished metallic surface. C the source of light, and the arrows the direction of the ray. Then D would represent the angle of incidence or contact, and E the angle of reflection or departure-which angles are seen to be equal.

C

D

E

508. What articles made with double-convex lenses? Uses? (Power of burning-glasses?)

504. What now shown in this chapter? What next?

505. What is reflection, and when does it take place? What law governs it? (Diagram.)

506. A concave mirror reflects parallel rays back to a focus, the distance of which is equal to half the radius of the sphere formed by the concave surface produced.

In this cut, the parallel rays A fall upon the concave mirror B B, and are reflected to the focus C, which is half the radius of the sphere formed by the surface of the mirror produced. If, therefore, it was desirable to construct a concave mirror, having its focus 10 feet distant, it would only be necessary to grind it on the circle of a sphere having a radius of 20 feet.

507. In reflection, a portion of the light is absorbed or otherwise lost, so that a reflector of a given diameter will not converge as much light to a focus as a double-convex lens of the same size. In the latter case, all the light is transmitted. Still, reflectors have been formed of such power as to melt iron, and other more difficult sub

stances.

We have now considered so much of optics as is necessary to an understanding of the principles upon which telescopes are constructed; and, for further particulars, shall refer the student to books of Natural Philosophy.

506. How does a concave mirror reflect parallel rays? Distance of focus? (Diagram. How would you construct a concave mirror with a 10 feet focus?) 507. Is all the light falling upon a polished surface reflected? What then? (Closing note?)

CHAPTER II.

TELESCOPES.

508. A Telescope is an optical instrument employed in viewing distant objects, especially the heavenly bodies. The term telescope is derived from two Greek words, viz., tele, at a distance, and skopeo, to see.

509. So far as is now known, the ancients had no knowledge of the telescope. Its invention, which occurred in 1609, is usually attributed to Galileo, a philosopher of Florence, in Italy.

The discovery of the principle upon which the refracting telescope is constructed was purely accidental. The children of one Jansen, a spectacle-maker of Middleburgh, in Holland, being at play in their father's shop, happened to place two glasses in such a manner, that in looking through them, at the weather-cock of the church, it appeared to be nearer and much larger than usual. This led their father to fix the glasses upon a board, that they might be ready for observation; and the news of the discovery was soon conveyed to the learned throughout Europe. Galileo hearing of the phenomenon, soon discovered the secret, and put the glasses in a tube, instead of on a board; and thus the first telescope was constructed.

510. The telescope of Galileo was but one inch in diameter, and magnified objects but 30 times. Yet with this simple instrument he discovered the face of the moon to be full of inequalities, like mountains and valleys; the spots on the sun; the phases of Venus; the satellites of Jupiter; and thousands of new stars in all parts of the heavens.

Notwithstanding this propitious commencement, so slow was the progress of the telescope toward its present state, that in 1816, Bonnycastle speaks of the 30-fold magnifying power of the telescope of Galileo as "nearly the greatest perfection that this kind of telescope is capable of!"

511. If he be the real author of an invention who, from a knowledge of the cause upon which it depends, deduces it from one principle to another, till he arrives

508. Subject of Chap. II.? 509. Ancient or modern?

Telescope? Derivation? Inventor? (Incidents of discovery?) 510. Galileo's telescope? Discoveries with it? (Progress in telescope making?)

511. Is Galileo entitled to the honor of inventing the telescope? (Whero is his ?)

at the end proposed, then the whole merit of the invention of the telescope belongs to Galileo. The telescope of Jansen was a rude instrument of mere curiosity, accidentally arranged; but Galileo was the first who constructed it upon principles of science, and showed the practical uses to which it might be applied.

It is said that the original telescope constructed by Galileo is still preserved in the British Museum. A pigmy, indeed, in its way, but the honored progenitor of a race of giants!

512. The discovery of the telescope tended greatly to sustain the Copernican theory, which had just been promulgated (10), and of which Galileo was an ardent disciple. Like Copernicus, however, his doctrines subjected hm to severe persecutions, and he was obliged to renounce them.

The following is his renunciation, made June 28, 1633: "I, Galileo, in the seventieth year of my age, on bended knees before your eminences, having before my eyes and touching with my hands the Holy Gospels, I curse and detest the error of the earth's movement." As he left the court, however, after this forced renunciation, he is said to have stamped upon the earth, and exclaimed, "It does move, after all!" Ten years after this he was sent to prison for the same supposed error; and soon, his age advancing, the grave received him from the malice of his persecutors.

DIFFERENT KINDS OF TELESCOPES.

513. Telescopes are of two kinds-Reflectors and Refractors. Refracting telescopes are made by refracting the light to a focus with a glass lens (499); and reflecting telescopes, by reflecting it to a focus with a concave mirror (506). Besides this general division, there are various kinds, both of reflectors and refractors.

514. Telescopes assist vision in various ways-first, by enlarging the visual angle under which a distant object is seen, and thus magnifying that object; and, secondly, by converging to a point more light than could otherwise enter the eye-thus rendering objects distinct or visible that would otherwise be indistinct or invisible.

All the light falling upon a six or a twelve inch lens may be converged to a focus, so as to be taken into the human eye through the pupil, which is but one-fourth of an inch in diameter. Our vision is thus made as perfect by art as if nature had given us ability to enlarge the eye till the pupil was a foot in diameter.

512. Relation of discovery to Copernican theory? Effects upon Galileo? (His renunciation? Death?)

513. Kinds of telescopes? Describe.

514. How assist vision? (Illustrative note?)