upon the position of the second prism, and in some positions one ray disappeared entirely. The light had been reduced to vibrations in one plane. In 1808, Malus, happening to direct a double refracting prism to the windows then reflecting the sunset, found that as he turned the prism round, the ordinary image of the window nearly disappeared in two opposite positions; and in two other positions, at right angles, the “extraordinary” image nearly vanished. So he found that polarization was produced by reflection as well as by transmission. The differences between common and polarized light have been summed up by Mr. Goddard as follows:— COMMON LIGHT POLARIZED LIGHT “Is capable of reflection at “Is capable of reflection at oblique angles of incidence in oblique angles only in every position of the reflector. _certain positions_ of the reflector. “Will pass through a bundle of “Will only pass through such plates of glass in any position glasses when they are in in which they may be placed. _certain positions_. “Will pass through a plate of “Will only pass in certain tourmaline, cut parallel to the positions, and in others axis of the crystal, in every will not pass at all.” position of the plate.” The bundle of glass plates or the tourmaline plate is thus the test for polarized light, and is termed an analyzer. The arrangement called a “Nichol’s prism,” made by cutting a prism of Iceland spar and uniting the halves with a cement, so that only one polarized ray can pass through it, is termed a _Polarizer_. It only permits one of the two rays produced by “double refraction” to pass, and the ray (as said above) will contain none but transverse vibrations. Polarized light will produce beautiful colours. The whole subject is very interesting to the scientist, but rather a difficult one for the general reader to understand. Amongst the uses to which light has been put is that of a milk-tester. The LACTOSCOPE will show the quantity of butter contained in a certain quantity of milk, by diluting it till it displays a certain degree of transparency. There is another method, by the transmission of light. The first test is obtained by means of a glass tube about nine inches long, closed at one end, and containing a small porcelain rod marked with black lines. A small quantity of milk is measured and placed in the tube. The black lines cannot at first be seen through the tube, but by adding water the milk is rendered transparent, and the black lines become visible. The surface of milk in the tube, by a graduated scale upon it, shows the percentage of butter. [Illustration: Fig. 167.—Cut card figures.]