dark bands at once disappear. It is therefore evident that the dark fringes have been caused by the interference of the two pencils when they have crossed, and that, as with sound, two systems of waves may interfere and destroy each other, producing silence, so with light, two systems interfere and produce darkness.

The same results are obtained with any other homogeneous light, the only difference being in the distances of the bands from each other. This experiment is regarded as giving the most satisfactory evidence of the correctness of the undulatory or wave hypothesis of light. In white light the component colors produce dark bands at different intervals, these being superimposed but not coincident; the dark lines of one color are illuminated by the others, and a series of colored bands arise.

Explain the saccharimeter and its use.

Saccharimeters are instruments depending upon polarized light for their action. That of Biot (Fig. 29) determines the amplitude of rota

tion produced by sugar or other solutions possessing this power. The polarizer in the original form is a mirror of black glass, and the analyzer a double refracting achromatic prism mounted in the interior of a graduated circle, so that changes in position of the prism are read by means of an index traversing the scale. Between these the tube containing the solution is placed, with its axis coincident with the conjoined axes of the analyzer and polarizer. The tube is about 20 centimetres in length. The apparatus is adjusted to have the extraordinary ray disappear when the index is at zero. The introduction of water, alcohol, or ether causes no change in the phenomena observed.

If it is then filled with a solution of cane-sugar, the extraordinary ray reappears, and to cause it to fade the analyzer must be turned to the right of zero through a certain number of degrees, which measures the amount that the sugar has turned the plane of polarization to the left. If a longer tube be used with a solution of the same strength, the analyzer must be turned through a greater number of degrees. According as the strength of the solution differs, so does the angle at which the analyzer must be set vary. It is therefore a simple matter to estimate the amount of sugar in any given solution by determining its power to rotate a beam of polarized light.

In the manufacture and refining of sugar the saccharimeter is important, as the sales of this commodity are based upon its indications. In the practice of medicine it is used for determining the percentage of sugar in diabetic urine and in other fluids.

The principle of polarization is further applied to examine the nature and purity of crystals, for the detection of false gems, and to determine whether the light from heavenly bodies is reflected or is light direct from self-luminous bodies.

What is the photographic camera?

The camera consists of an achromatic lens mounted in a wooden box, at the back of which is a ground-glass plate for the reception of the image projected by the object before the lens. The image is focussed on the ground-glass, which is then removed and replaced by a "holder" containing a sensitized plate. Light is then admitted for an instant through the lens, and the background of the image is darkened, while the image itself remains light. This plate is called the negative, because when placed over sensitized paper and exposed to the sun it admits light through the image and excludes it from the background, thus making the colors on the paper the reverse of those on the glass. Explain the stereopticon.

The stereopticon is the converse of the camera. It is used to project magnified images on a screen in a darkened room. A transparency or slide, produced by the camera, is placed between the two lenses of a stereopticon and its image, inverted and enlarged, is produced on the

screen.

This is accomplished by confining a light within the stereopticon and causing its emitted rays to pass through a convex or condensing lens, thence through the transparency, and lastly through a movable focussing lens which inverts and magnifies the shadow or image.

Describe the stereoscope.

The stereoscope is an instrument by which two pictures of the same object taken at a proper angle may appear as one. Each eye sees only the picture opposite it, but by suitable contrivances the two images are made to coincide and appear in the same place.

2

A

FIG. 30.

B

P

P2

Let the left eye be at B (Fig. 30), and the right at A; let a and b be the corresponding pictures for each eye, and P and P two prisms of glass through which they are seen. A prism refracts rays of light so that objects viewed through it appear to be nearer to the refracting edge; the prism P, therefore, refracts the ray aP in the direction PA, as if it proceeded from c. The prism P, refracts the ray bP2, so that to the eye at B it also appears to proceed from c. The effect of this is-provided that the two pictures a and b are drawn just as a body at c would appear to the eyes at A and B if the prisms were not there-that the object really appears to be at c. And as the points a and b combine to form the point c, so d and e unite to form the point f, g and h to form the point i.

a

The stereoscope.

Similar effects of relief may be produced by the projection on the same spot of two photographic images of any object taken at a suitable angle to each other.

How do we estimate the size and distance of objects?

Our estimation of these depends: 1st, upon comparison with those of known size in the same locality; 2d, upon the visual angle; 3d, upon the optic angle. Regarding the latter, Ganot says: "This angle increasing or diminishing according as objects approach or recede, we move our eyes so as to make their optic axes converge towards the object which we are looking at, and thus obtain an idea of its distance. Nevertheless, it is only by long custom that we can establish a relation between our distance from the objects and the corresponding motion of the eyes. It is a curious fact that persons born blind, but whose sight has been restored by an operation for cataract, imagine at first that all objects are at the same distance.

17-C. & P.

What is the principle of the compound microscope?

The principle of the compound microscope is to produce an image by the aid of a lens and to magnify this image by one or more lenses before it reaches the eye. Suitable adjustments for focussing and illuminating the object are a part of the instrument. The microscope is used to examine near objects too small to be seen distinctly by the unaided eye.

What is the use of the telescope?

The telescope is used to view objects which appear small on account of their distance. It consists of an object glass which forms a real image, inverted but small. This image is viewed through a microscope and seen in an inverted position. If the telescope be used for terrestrial purposes, two additional lenses must be inserted between the eye piece and the object lens in order to bring the image to an upright position. How is the intensity of light measured?

The intensity of light is measured by comparison. The standard is called candle-power. It is the light given by an English sperm-candle burning at the rate of 120 grains per hour. A transparent disc is illumined on one side by the standard candle and on the opposite side by the light to be tested. The disc is then slowly moved along a graduated scale which joins the two lights until both sides are equally

illuminated. The distance to each light is then observed, and the intensity computed on the principle that the intensity of light is inversely as the square of the distance. Rumford's photometer is an opaque rod placed at a short distance from a screen; the two sources of light are then arranged, as in Fig. 31, so that each forms a shadow of the rod

upon it. The lights are then moved until the shadows are of equal intensity. Since that from the lamp is illuminated by the candie, and that from the candle by the lamp, the illumination of the screen by the sources of light is equal. The intensities of the lights are, therefore, as the squares of their distances from the screen.

MAGNETISM.

What is a natural magnet?

Lodestone is the only natural magnet known. It is an oxide of iron (Fe,O,), or often a compound of two oxides, and it occurs as an ore, fragments of which, when suspended, tend to point north and south. They were therefore called leading stones or lodestone. The Greeks applied to the ore the name magnus, from Magnesia, the city near which it was found. Its property, termed magnetism, is to attract iron.

What are artificial magnets?

They are made from a few substances which are capable of receiving magnetism. Of the eligible substances, such as cobalt, nickel, and steel, the latter is almost invariably used.

What are the varieties of artificial magnets?

Bar magnets, horseshoe magnets (Fig. 32), and magnetic needles. Other shapes may be made, but these are the most convenient forms.

Is the magnetism of a magnet permanent?

Much depends on the material and the shape of the magnet. Those called permanent are made, but in time their power is enfeebled or totally lost. A piece of soft iron, an armature, joining the ends of the horseshoe, guards against the decrease of force.

What is a compound magnet?

Several bars or horseshoes, separately magnetized and riveted together, give better effects than the same amount of iron in one magnet. When great force is required, this compound form (Fig. 33) is used.

What are the properties of magnets?

FIG. 32.

B

N

S

K

Attraction and repulsion. Substances of one class, such as iron, steel, cobalt, nickel, and platinum, are Horseshoe magnet. more or less easily attracted, and are therefore termed

paramagnetic bodies; those of another class, such as bismuth, antimony, zinc, tin, mercury, lead, silver, copper, gold, and arsenic, are