Читать книгу Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc - Gaston Tissandier - Страница 17

Common Light	Polarized Light
“Is capable of reflection at oblique angles of incidence in every position of the reflector.	“Is capable of reflection at oblique angles only in certain positions of the reflector.
“Will pass through a bundle of plates of glass in any position in which they may be placed.	“Will only pass through such glasses when they are in certain positions.
“Will pass through a plate of tourmaline, cut parallel to the axis of the crystal, in every position of the plate.”	“Will only pass in certain positions, and in others will not pass at all.”

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.

We are all so familiar with the “Magic Lantern,” and the apparatus for dissolving views by an arrangement of lenses and manipulation of slides, that we need do no more than refer to them.

Fig. 169.—Dissolving views apparatus.

The various ghost illusions, etc., produced by indirect mirrors, have already been referred to, the ghost being merely the reflection of an individual seen through a sheet of glass between the spectators and the stage. The strong light throws a reflection from a parallel mirror lower down, and this reflected image can be made to appear amongst the real actors who are behind the plate-glass in full view of the audience, who are, however, ignorant of the existence of the glass screen.

For the winter evenings one may easily procure an apparatus for dissolving views by the oxy-hydrogen light. One, as shown in the illustration herewith (fig. 169), will answer every purpose, and by this double arrangement phantasmagoria may be produced, or a fairy tale may be illustrated. The effect of gradually-approaching night may be given to the picture by means of a special glass in the lower lanthorn. The apparatus is exhibited by means of a Drummond light, and is very simple, although a certain supply of gas is necessary for the performance. But this can be easily procured by an indiarubber tube, or in a bag supplied for the purpose. Almost any objects can be used, photographs, etc., etc., and many very comical arrangements can be made.

We have lately been reading a curious method of obtaining light from oyster-shells in a Trans-Atlantic magazine. We give an extract wherewith to close this chapter. The compound is “luminous paint.”

“It has been known that certain compounds of lime and sulphur had the property of absorbing light, and giving it out again when placed in the dark. A simple way to do this is to expose clean oyster-shells to a red heat for half an hour. When cold, the best pieces are picked out and packed with alternate layers of sulphur in a crucible, and exposed to a red heat for an hour. When cold, the mass is broken up, and the whitest pieces are placed in a clean glass bottle. On exposing the bottle to bright sunshine during the day, it is found that at night its contents will give out a pale light in the dark. Such a bottle filled more than a hundred years ago still gives out light when exposed to the sun, proving the persistency of the property of reproducing light. Very many experiments have been more recently made in this direction, and the light-giving property greatly enhanced. The chemicals, ground to a flour, may now be mixed with oils or water for paints, may be powdered on hot glass, and glass covered with a film of clear glass, or mixed with celluloid, papier-maché, or other plastic materials. As a paint, it may be applied to a diver’s dress, to cards, clock dials, sign-boards, and other surfaces exposed to sunlight during the day; the paint gives out a pale violet light at night sufficient to enable the objects to be readily seen in the dark. If the object covered with the prepared paint is not exposed to the sun, or if the light fades in the dark, a short piece of magnesium wire burned before it serves to restore the light-giving property.”