34. (Pg. 155) How is a musical tone produced? 35. (Pg. 155) What occasions discords? 36. (Pg. 155) Upon what does the acuteness or gravity of a sound depend? 37. (Pg. 155) Does the force, with which a string is struck, affect the rapidity of its vibrations? 38. (Pg. 155) How are the strings made to produce the high and low notes? 39. (Pg. 155) What is meant by harmony, or concord, and how is it produced? 40. (Pg. 156) When are strings said to be in unison? 41. (Pg. 156) How are octaves produced? 42. (Pg. 156) How are fifths produced? 43. (Pg. 156) How major and minor thirds? 44. (Pg. 156) What is meant by melody, and in what particular does it differ from harmony? [Illustration: PLATE XV.] CONVERSATION XIV. ON OPTICS. OF LUMINOUS, TRANSPARENT, AND OPAQUE BODIES. OF THE RADIATION OF LIGHT. OF SHADOWS. OF THE REFLECTION OF LIGHT. OPAQUE BODIES SEEN ONLY BY REFLECTED LIGHT. VISION EXPLAINED. CAMERA OBSCURA. IMAGE OF OBJECTS ON THE RETINA. 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. Light, when emanated from the sun, or any other luminous body, is projected forward in straight lines, in every possible direction; so that the luminous body, is not only the general centre, from whence all the rays proceed; but every point of it, may be considered as a centre, which radiates light in every direction. (Fig. 1, plate 15.) _Emily._ But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course?