Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

Caroline. Yet I see the sun, shining on that house yonder, as clearly as possible.

Mrs. B. Indeed you cannot see a single ray, which passes from the sun to the house; you see, by the aid of those rays, which enter your eyes; therefore, it is the rays which are reflected by the house, to you, and not those which proceed directly from the sun, to the house, that render the building visible to you.

Caroline. Why then does one side of the house appear to be in sunshine, and the other in shade? for, if I cannot see the sun shine upon it, the whole of the house should appear in the shade.

Mrs. B. That side of the house, which the sun shines upon, receives, and reflects more light, and therefore, appears more luminous and vivid, than the side which is in shadow; for the latter is illumined only, by rays reflected upon it by other objects; these rays are, therefore, twice reflected before they reach your sight; and as light is more, or less, absorbed by the bodies it strikes upon, every time a ray is reflected, its intensity is diminished.

Caroline. Still I cannot reconcile to myself, the idea that we do not see the sun's rays shining on objects, but only those which such objects reflect to us.

Mrs. B. I do not, however, despair of convincing you of it. Look at that large sheet of water; can you tell why the sun appears to shine on one part of it only?

Caroline. No, indeed; for the whole of it is equally exposed to the sun. This partial brilliancy of water, has often excited my wonder; but it has struck me more particularly by moonlight. I have frequently observed a vivid streak of moonshine on the sea, while the rest of the water remained in deep obscurity, and yet there was no apparent obstacle to prevent the moon from shining equally on every part of the water.

Mrs. B. By moonlight the effect is more remarkable, on account of the deep obscurity of the other parts of the water; while by the sun's light, the effect is too strong for the eye to be able to observe it so distinctly.

Caroline. But, if the sun really shines on every part of that sheet of water, why does not every part of it, reflect rays to my eyes?

Mrs. B. The reflected rays, are not attracted out of their natural course, by your eyes. The direction of a reflected ray, you know, depends on that of the incident ray; the sun's rays, therefore, which fall with various degrees of obliquity upon the water, are reflected in directions equally various; some of these will meet your eyes, and you will see them, but those which fall elsewhere, are invisible to you.