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.
Fig. 167.—Cut card figures.
The second method is not so simple. A short tube of tin, blackened on the inside, and supported upright, has an opening on one side, and opposite this, inside the tube, is a mirror placed at an angle of 45°. “By placing a lighted candle at a known distance opposite the opening, its light is reflected in the mirror and thrown upward through the tube. On top of the tube is placed a round vessel of glass or metal, closed at the bottom by a sheet of clear glass. The vessel is closed at the top by a cover having an opening in the centre, in which slides up and down a small tube closed at the bottom with glass, and having an eye-piece at the top. The milk to be tested is placed in this vessel on the top of the tin tube, so that the light of the candle reflected from the mirror passes upward through the milk. Then, by looking through the sliding tube and moving it up and down, a point may be found where the image of the candle in the mirror can be seen through the milk. This device depends, as will be seen, on observing the light transmitted through a film of milk, and the thickness of the film is the measure of the value of the milk. The movable tube contains a graduated scale, and by comparison of this with a printed table, the percentage of butter in the milk may be ascertained.”
In concluding this chapter we give a few hints for some pleasant relaxation for young people, which has many a time created amusement. The experiment consists in cutting out in paper or cardboard certain portions of a face or figure, as per the illustration herewith. Fig. 167 gives the card as cut with the scissors, and the two subsequent faces are the result of the same held at a less or greater distance from a screen. The illustrations (fig. 168) will assist those who wish to amuse children by making rabbits, etc., on the wall. The shadows will be seen perfectly thrown if the hands be carefully fixed near a good light.
Fig. 168.—Hand-shadows on the wall.