not only very plain, but self-evident. For if light be progressive, the position of the telescope, in order to receive the ray, must be different from what it would have been if light had been instantaneous, or if the Earth stood still. Hence the place

to which the telescope is directed will be different from the true place of the object.

The quantity of this aberration is determined by a simple proposition. The Earth describes 59' 8" of her orbit in a day =3548", and a ray of light comes from the Sun to us in 8′ 13′′ =493": now 24 hours or 86400": 493: : 3548: 22"; which is the change in the star's place, arising from the cause abovementioned.

CHAPTER XVIII.

PRACTICAL ASTRONOMY-REFLECTION AND REFRACTION OF LIGHT.

667. Practical Astronomy has respect to the means employed for the acquisition of astronomical knowledge. It includes the properties of light, the structure and use of instruments, and the processes of mathematical calculation.

In the present treatise, nothing further will be attempted than a mere introduction tc practical astronomy. In a work designed for popular use, mathematical demonstrations would be out of place. Still, every student in astronomy should know how telescopes are made, upon what laws they depend for their power, and how they are used. It is for this purpose mainly that we add the following chapters on practical astronomy.

PROPERTIES OF LIGHT.

668. Light is that invisible ethereal substance by which we are apprised of the existence, forms, and colors of materia! objects, through the medium of the visual organs. To this subtile fluid we are especially indebted for our knowledge of those distant worlds that are the principal subjects of astronomical inquiry.

It may

669. The term light is used in two different senses. signify either light itself, or the degree of light by which we are enabled to see objects distinctly. In this last sense, we put light

the quantity of aberration determined? 667. Subject of Chapter XVIII. What is practical astronomy? How far discussed in this treatise? 668. Define light. For 669. Different senses in which the term is used? What is

what indebted to it?

in opposition to darkness. But it should be borne in mind, that durkness is merely the absence of that degree of light which is necessary to human vision; and when it is dark to us, it may be light to many of the lower animals. Indeed, there is more or less light, even in the darkest night, and in the deepest dungeon.

"Those unfortunate individuals," says Dr. Dick, "who have been confined in the darkest dungeons, have declared, that though, on their first entrance, no object could be perceived, perhaps for a day or two, yet, in the course of time, as the pupils of their eyes expanded, they could readily perceive rats, mice, and other animals that infested their cells, and likewise the walls of their apartments; which shows that, even in such situations, light is present, and produces a certain degree of influence."

670. Of the nature of the substance we call light, two theo ries have been advanced. The first is, that the whole sphere of the universe is filled with a subtile fluid, which receives from luminous bodies an agitation; so that, by its continued vibratory motion, we are enabled to perceive luminous bodies. This was the opinion of Descartes, Euler, Huygens, and Franklin.

The second theory is, that light consists of particles thrown off from luminous bodies, and actually proceeding through space. This is the doctrine of Newton, and of the British philosophers generally.

Without attempting to decide, in this place, upon the relative merits of these two hypotheses, we shall use those terms, for convenience sake, that indicate the actual passage of light from one body to another.

671. Light proceeds from luminous bodies in straight lines, and in all directions. It will not wind its way through a crooked passage, like sound; neither is it confined to a part of the circumference around it.

As the Sun may be seen from every point in the solar system, and far hence into space 'n every direction, even till he appears but a faint and glimmering star, it is evident that he fills every part of this vast space with his beams. And the same might be said of every star in the firmament.

672. As vision depends not upon the existence of light merely. but requires a certain degree of light to emanate from the object, and to enter the pupil of the eye, it is obvious that if we can, by any means, concentrate the light, so that more may enter the eye, it will improve our perception of visible objects, and even enable us to see objects otherwise wholly invisible.

Some animals have the power of adapting their eyes to the existing degree of light. The cat, horse, &c., can see day or night; while the owl, that sees well in the night, sees poorly in the day-time.

673. Light may be turned out of its course either by reflection

dark'iess?

Can it be dark and light at the same time? Is there any place without light? Quotation from Dr. Dick? 670. What theories of the nature of light, and by whom supported respectively? Remark of author? 671. How light proceeds from uminous bodies? Radiations from Sun and stars? 672. How improve vision, and 678. How is light turned out of its course?

why? Animals ?

or refraction. It is reflected when it falls upon the highly polished surface of metals and other intransparent substances; and refracted when it passes through transparent substances of different densities, as already illustrated in Chapter XVI.

REFRACTION BY GLASS LENSES.

674. A lens is a piece of glass, or other transparent substance, of such a form as to collect or disperse the rays of light that are passed through it, by refracting them out of a direct course. They are of different forms, and have different powers.

In the adjoining cut, we have an edgewise view of six different lenses.

A is the plano-convex, or half a double convex lens; one side being convex and the other plane.

B is a plano-concave; one surface being concave, and the other plane.

C is a double-convex lens, or one that is bounded by two convex surfaces.

D is a double-concave lens, or a circular piece of glass hollowed out on both sides.

E is a concavo-convex lens, whose curves differ, but do not meet, if produced.

F is a meniscus, or a concavo-convex lens, the curves of whose surfaces meet.

675. A double-convex lens converges parallel rays to a point called the focus; and the distance of the focus depends upon the degree of corvexity.

In the first of these cuts, the lens is quite thick, and the focus of the rays is quite near; but the other being less convex, the focus is more distant.

676. The distance of the focus of a double-convex glass lens is the radius of the sphere of its convexity.

In this cut, it will be seen that the parallel rays A are refracted to a focus at C, by the double-convex lens B, the convexity of whose surfaces is just equal to the curve of the circle D.

677. The focal distance of a plano-convex lens is equal to the diameter of the sphere

A

LENSES OF DIFFERENT FORMS.

B C D

E F

MOXCO

LIGHT REFRACTED BY LENSES.

DOUBLE CONVEX-FOCAL DISTANCE.

formed by the convex surface produced.

C

B

674. What is a lens? Draw and describe different kinds. 675. Refracting power of double convex lens? Focal distance? Diagram, and illustrate. 676. Hew focal distance governed? Diagram, and illustrate. 677. What is the focal distance of a

It must be borne in mind, that iight 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 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 C B is the diameter of the circle D, formed by the convex surface of the lens C produced.

678. A double-con

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

In this cut, the parallel rays A are dispersed by the doubleconcave lens C, as shown at B; and their direction, as thus refracted, is the same

RAYS DISPERSED BY REFRACTION.

B

as if they proceeded from the point D, which is the center of a circle formed by the concave surface of the lens produced.

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

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 substances, as platinum, agate, &c. "A lens three feet in diameter," says Professor Gray, "has been known to melt cornelian in 75 seconds, and a piece of white agate in 30 seconds."

BURNING-GLASS.

plano-convex lens? Diagram. 678. Effect of double-convex lens?
geucy of rays?
679. What articles made with double-convex lenses?
of burning glasses?

Amount of diver

Uses? Powel

REFLECTION OF LIGHT.

680. 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.

REFEECTION BY A PLANE MIRROR.

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.

681. 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 fail 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.

682. In reflection, a portion of the light is absorbed or otherwise lost, so that a reflector of a given diameter

B

B

E

B

REFLECTION BY A CONCAVE MIRROR.

-D

F

E

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 found of such power as to melt iron, and other more difficult substances.

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 on Natural Philosophy.

680. What now shown in this chapter? What next? What is reflection, and when does it take place? What law governs it? Diagram. 681. How does a concave mirror reflect parallel rays! Distance of focus? Diagram. How would you construct a concave mirror with a 10 feet focus ? 682. Is all the light falling upon a polished surface reflected? What then? Closing note?