CAROLINE.
I long to begin our lesson to-day, Mrs. B., for I expect that it will be very entertaining.
Mrs. B. Optics is that branch of philosophy, which treats of the nature and properties of light. It is certainly one of the most interesting branches of Natural Philosophy, but not one of the easiest to understand; I must, therefore, beg that you will give me your undivided attention.
I shall first inquire, whether you comprehend the meaning of a luminous body, an opaque body, and a transparent body.
Caroline. A luminous body is one that shines; an opaque....
Mrs. B. Do not proceed to the second, until we have agreed upon the definition of the first. All bodies that shine, are not luminous; for a luminous body is one that shines by its own light; as the sun, the fire, a candle, &c.
Emily. Polished metal then, when it shines with so much brilliancy, is not a luminous body?
Mrs. B. No, for it would be dark, if it did not receive light from a luminous body; it belongs, therefore, to the class of dark, as well as of opaque bodies, which comprehends all such as are neither luminous, nor will admit the light to pass through them.
Emily. And transparent bodies, are those which admit the light to pass through them, such as glass and water.
Mrs. B. You are right. Transparent, or pellucid bodies, are frequently called mediums, because they allow the rays of light to pass through them; and the rays which pass through, are said to be transmitted by them.