Mrs. B. Precisely. Emily. But is that side of the house yonder, which appears to be in shadow, really illumined by the sun, and its rays reflected another way? Mrs. B. No; that is a different case from the sheet of water. That side of the house is really in shadow; it is the west side, which the sun cannot shine upon till the afternoon. Emily. Those objects, then, which are illumined by reflected rays, and those which receive direct rays from the sun, but which do not reflect those rays towards us, appear equally in shadow ? Mrs. B. Certainly; for we see them both illumined by reflected rays. That part of the sheet of water, over which the trees cast a shadow, by what light do you see it. Emily. Since it is not by the sun's direct rays, it must be by those reflected on it from other objects, and which it again reflects to us. Caroline. But if we see all terrestrial objects by reflected light, (as we do the moon,) why do they appear so bright and luminous ? I should have supposed that reflected rays would have been dull and faint, like those of the moon. Mrs. B. The moon reflects the sun's light with as much vividness as any terrestrial object. If you look at it on a clear night, it will appear as bright as a sheet of water, the walls of a house, or any object seen by daylight and on which the sun shines. The rays of the moon are doubtless feeble, when compared with those of the sun; but that would not be a fair comparison, for the former are incident, the latter reflected rays. Caroline. True ? and when we see terrestrial objects by moon-light, the light has been twice reflected, and is consequently proportionally fainter. Mrs. B. In traversing the atmosphere, the rays, both of the sun and moon, lose some of their light. For though the pure air is a transparent medium, which transmits the rays of light freely, we have observed, that near the surface of the earth it is loaded with vapors and exhalations, by which some portion of them are absorbed. Caroline. I have often noticed that an object on the summit of a hill appears more distinct than one at an equal distance in a valley, or on a plain; which is owing, I suppose, to the air being more free from vapors in an elevated situation, and the reflected rays being consequently brighter. Mrs. B. That may have some sensible effect; but when an object on the summit of a hill has a back ground of light sky, the contrast with the object makes its outline more distinct. Caroline. I now feel well satisfied, that we see opaque objects only by reflected rays; but I do not understand how these rays show us the objects from which they proceed? Mrs. B. The rays of light enter at the pupil of the eye, and proceed to the retina, or optic nerve, which is situated at the back part of the eye-ball; and there they describe the figure, color, and (excepting size) form a perfect representation of the object from which they proceed. We shall again close the shutters, and admit the light through the small aperture, and you will see a picture on the wall, opposite the aperture, similar to that which is delineated on the retina of the eye. Caroline. Oh, how wonderful! There is an exact picture in miniature of the garden, the gardener at work, the trees blown about by the wind. The landscape would be perfect, if it were not reversed; the ground being above, and the sky beneath. Mrs. B. It is not enough to admire, you must understand this phenomenon, which is called a camera obscura, from the necessity of darkening the room, in order to exhibit it. This picture is produced by the rays of light reflected from the various objects in the garden, and which are admitted through the hole in the window shutter. The rays from the glittering weathercock at the top of the alcove A, (plate XVI. fig. 1.) represent it in this spot a; for the weathercock heing much higher than the aperture in the shutter, only a few of the rays, which are reflected by it in an obliquely descending direction, can find entrance there. The rays of light, you know, always move in straight lines; those, therefore, which enter the room in a descending direction, will continue their course in the same direction, and will, consequently, fall upon the lower part of the wall opposite the aperture, and represent the weathercock reversed in that spot, instead of erect in the uppermost part of the landscape. Emily. And the rays of light from the steps (B) of the alcove, in entering the aperture, ascend, and will describe those steps in the highest instead of the lowest part of the landscape. Mrs. B. Observe, too, that the rays coming from the alcove, which is to our left, describe it on the wall to the right; while those which are reflected by the walnuttree C D, to our right, delineate its figure in the picture to the left c d. Thus the rays, coming in different |