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.