As the science of astronomy depends solely on the influence of light upon the organ of vision, which is the most noble and extensive of all our senses; and as the construction of telescopes and other astronomical instruments is founded upon our knowledge of the nature of light and the laws by which it operates—it is essentially requisite, before proceeding to a description of such instruments, to take a cursory view of its nature and properties, in so far as they have been ascertained, and the effects it produces when obstructed by certain bodies, or when passing through different mediums.
CHAPTER I.
GENERAL PROPERTIES OF LIGHT.
It is not my intention to discuss the subject of light in minute detail—a subject which is of considerable extent, and which would require a separate treatise to illustrate it in all its aspects and bearings. All that I propose is to offer a few illustrations of its general properties, and the laws by which it is refracted and reflected, so as to prepare the way for explaining the nature and construction of telescopes, and other optical instruments.
There is no branch of natural science more deserving of our study and investigation than that which relates to light—whether we consider its beautiful and extensive effects—the magnificence and grandeur of the objects it unfolds to view—the numerous and diversified phenomena it exhibits—the optical instruments which a knowledge of its properties has enabled us to construct—or the daily advantages we derive, as social beings, from its universal diffusion. If air, which serves as the medium of sound, and the vehicle of speech, enables us to carry on an interchange of thought and affection with our fellow-men; how much more extensively is that intercourse increased by light, which presents the images of our friends and other objects as it were immediately before us, in all their interesting forms and aspects—the speaking eye—the rosy cheeks—the benevolent smile, and the intellectual forehead! The eye, more susceptible of multifarious impressions than the other senses, ‘takes in at once the landscape of the world,’ and enables us to distinguish, in a moment, the shapes and forms of all its objects, their relative positions, the colours that adorn them, their diversified aspect, and the motions by which they are transported from one portion of space to another. Light, through the medium of the eye, not only unfolds to us the persons of others, in all their minute modifications and peculiarities, but exhibits us to ourselves. It presents to our own vision a faithful portrait of our peculiar features behind reflecting substances, without which property we should remain entirely ignorant of those traits of countenance which characterize us in the eyes of others.
But, what is the nature of this substance we call light, which thus unfolds to us the scenes of creation? On this subject two leading opinions have prevailed in the philosophical world. One of those opinions is, that the whole sphere of the universe is filled with a subtle matter, which receives from luminous bodies an agitation which is incessantly continued, and which, by its vibratory motion, enables us to perceive luminous bodies. According to this opinion, light may be considered as analogous to sound, which is conveyed to the ear by the vibratory motions of the air. This was the hypothesis of Descartes, which was adopted, with some modifications, by the celebrated Euler, Huygens, Franklin, and other philosophers, and has been admitted by several scientific gentlemen of the present day. The other opinion is, that light consists of the emission or emanation of the particles of luminous bodies, thrown out incessantly on all sides, in consequence of the continued agitation it experiences. This is the hypothesis of the illustrious Newton, and has been most generally adopted by British philosophers.
To the first hypothesis, it is objected that, if true, ‘light would not only spread itself in a direct line, but its motion would be transmitted in every direction like that of sound, and would convey the impression of luminous bodies in the regions of space beyond the obstacles that intervene to stop its progress.’ No wall or other opaque body could obstruct its course, if it undulated in every direction like sound; and it would be a necessary consequence, that we should have no night, nor any such phenomena as eclipses of the sun or moon, or of the satellites of Jupiter and Saturn. This objection has never been very satisfactorily answered. On the other hand, Euler brings forward the following objections against the Newtonian doctrine of emanation. 1. That, were the sun emitting continually, and in all directions, such floods of luminous matter with a velocity so prodigious, he must speedily be exhausted, or at least, some alteration must, after the lapse of so many ages, be perceptible. 2. That the sun is not the only body that emits rays, but that all the stars have the same quality; and as every where the rays of the sun must be crossing the rays of the stars, their collision must be violent in the extreme, and that their direction must be changed by such a collision.2
To the first of these objections it is answered—that so vast is the tenuity of light, that it utterly exceeds the power of conception: the most delicate instrument having never been certainly put in motion by the impulse of the accumulated sun-beams. It has been calculated that in the space of 385,130,000 Egyptian years, (of 360 days,) the sun would lose only the 1/1,217,420th of his bulk from the continual efflux of his light. And, therefore, if in 385 millions of years the sun’s diminution would be so extremely small, it would be altogether insensible during the comparatively short period of five or six thousand years. To the second objection it is replied—that the particles of light are so extremely rare that their distance from one another is incomparably greater than their diameters—that all objections of this kind vanish when we attend to the continuation of the impression upon the retina, and to the small number of luminous particles which are on that account necessary for producing constant vision. For it appears, from the accurate experiments of M. D’Arcy, that the impression of light upon the retina continues eight thirds, and as a particle of light would move through 26,000 miles in that time, constant vision would be maintained by a succession of luminous particles twenty-six thousand miles distant from each other.
Without attempting to decide on the merits of these two hypotheses, I shall leave the reader to adopt that opinion which he may judge to be attended with the fewest difficulties, and proceed to illustrate some of the properties of light:—and in the discussion of this subject, I shall generally adhere to the terms employed by those who have adopted the hypothesis of the emanation of light.
1. Light emanates or radiates from luminous bodies in a straight line. This property is proved by the impossibility of seeing light through bent tubes, or small holes pierced in metallic plates placed one behind another, except the holes be placed in a straight line. If we endeavour to look at the sun or a candle through the bore of a bended pipe, we cannot perceive the object, nor any light proceeding from it, but through a straight pipe the object may be perceived. This is likewise evident from the form of the rays of light that penetrate a dark room, which proceed straight forward in lines proceeding from the luminous body; and from the form of the shadows which bodies project, which are bounded by right lines passing from the luminous body, and meeting the lines which terminate the interposing body. This property may be demonstrated to the eye, by causing light to pass through small holes into a dark room filled with smoke or dust. It is to be understood, however, that in this case, the rays of light are considered as passing through the same medium; for when they pass from air into water, glass, or other media, they are bent at the point where they enter a different medium, as we shall afterwards have occasion to explain.
2. Light moves with amazing velocity. The ancients believed that it was propagated from the sun and other luminous bodies instantaneously;