a. The mirror. b. The convex lens. c. The white table.
The term "focusing," or the art of moving the lenses so that a sharp image may be obtained, has been frequently mentioned in this article, and perhaps it may be as well to describe the mode of ascertaining the focal distance of a lens by experiment.
Hold the lens opposite the window so that a bright picture of the window-sash may be obtained on a sheet of paper pinned against the wall, and the distance of the lens from the paper will be the focal length.
If the lens has a very long focal length, it may be determined as follows:—Measure the distance between the lens and the object, and also from the image; multiply these distances together, and divide the product by their sums; the quotient will give the focal distance.
VI. The Decomposition of Light—"its Analysis and Synthesis."
It is in the Italian language that the bride, the emblem of purity, is called Lucia (Lux, light); and surely if an illustration were required of beauty and singleness, light would be named poetically as appropriate; but physically it is not of a single nature, it is composite, and made up of seven colours. The instrument required to refract a ray of light sufficiently to break it into its elementary colours is called the prism, and is a solid having two plane surfaces, called its refracting surfaces, with a base equally inclined to them. (Fig. 301.)
Fig. 301.
The prism. The base, a b, is equally inclined to the refracting surfaces, c a, c b.