A "dark chamber" is the name of a most amusing, and now, in the improved form, extremely valuable instrument for photographic purposes. It is occasionally to be met with in public gardens, and there is a very good one on the Hoe at Plymouth. The construction of the camera for observing the surrounding country is very simple, and merely consists of a flat mirror placed at an angle, by which the picture is reflected through a double-convex lens on to a white table beneath. (Fig. 300.) [Illustration: Fig. 300. A. The mirror. B. The convex lens. C. The white table.] [Page 309] 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.) [Illustration: Fig. 301. The prism. The base, A B, is equally inclined to the refracting surfaces, C A, C B.] It was in 1672 that Sir Isaac Newton made his celebrated analysis of light, by receiving a sunbeam (as it passed through a hole in a shutter) on to the refracting surface of a prism, and throwing the image or spectrum on to a screen, where he observed the seven colours, red, orange, yellow, green, blue, indigo, and violet, and thus proved "_that there are different species of light, and that each species is disposed both to suffer a different degree of refrangibility in passing out of one medium into another, and to excite in us the idea of a different colour from the rest; and that bodies appear of that colour which arises from the composition of those colours the several species they reflect are disposed to excite_."