466. The Stereoscope. Simultaneous vision with two eyes is supposed to give us the idea of relief, or form of objects, a view which receives confirmation from the action of

the stereoscope.

This is an apparatus employed to give to flat pictures the appearance of relief, that is, the appearance of having three dimensions.

When we look at an object with both eyes, each eye sees a slightly different portion of it. Thus, if we look at a small cube, as a die, for example, first with one eye and then with the other, the head remaining fast, we shall observe that the perspective of the cube is different in the two cases. This will be the more apparent the nearer the body.

If the cube has one face directly in front of the observer, and the right eye is closed, the other eye will see the front face and also the left-hand face, but not the right; if, however, the left eye is closed, the other eye will see the front face and also the right-hand face, but not the left. Hence we know that the two images formed by the two eyes are not absolutely alike. It is this difference of images which gives the idea of relief in looking at a solid body.

A

Fig. 321.

B

If, now, we suppose two pictures to be made of an object, the one as it would appear to the right eye and the other as it would appear to the left eye, and then look at them with both eyes through lenses that cause the pictures to coincide, the impression is precisely the same as though the object itself were before the eyes. The illusion is so complete that it is almost impossible to believe that we are simply viewing pictures on a flat surface.

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Such is the theory of the stereoscope. Fig. 321 shows the course of the rays in this instrument as just described. A represents a picture of the object as it would be seen by the right eye alone; B, a pieture of the same object as it would be seen by the left eye alone; m and n are lenses which deviate the rays so as to make the pictures appear to be coincident at C.

The lenses m and ʼn ought to be perfectly symmetrical, and this result is attained by cutting a double-convex lens in two, and placing the right-hand half before the left eye, and the other half before the right eye. The pictures must be perfectly executed, which can be done only by means of the photographic process. The pictures are made by using two cameras inclined to each other in the proper · angle.

Summary.

The Camera Obscura.

Construction.

Formation of the Image explained by Figure.

Result when the Aperture is Large.

When the Aperture is Small the Image is independent of its Shape.

Illustration of this fact by Figure.

Camera and Lens.

Its Value in forming Distinct Images.

Artist's Camera.

Portable Camera for Artists illustrated by Figure.

Course of the Rays of Light illustrated by Figure.

Photographer's Camera.

Construction and Method of using it explained by
Figure.

Positive and Negative Pictures defined.

Positive made from the Negative.

The Eye.

Defects as an Optical Instrument.

Parts of the Eye shown by Figure.

Description of the Various Parts of the Eye.

Mechanism of Vision.

Inversion of Image on the Retina.

Theories regarding it.

Distinct Vision.

Adjustment of the Eye to Distance.

Adjustment of the Eye to Different Degrees of Intensity.

Duration of the Impressions on the Retina.

Examples.

Near-sightedness and Far-sightedness.

Definition of the Terms.

Causes.

Vision with two Eyes.

Explanation.

The Stereoscope.

Definition.

Illustrations of the Principle.

Construction explained by Figure.

467. Nature of Electricity. - The real nature of electricity is difficult to determine. It manifests itself chiefly in attractions and repulsions, but it is also recognized by its luminous and heating effects, by its power in chemical decompositions, and, at times, by the violence of its action.

All electricity has the characteristic of polarity, or twosidedness, and is now generally conceded to be due to molecular motions. Several theories have been advanced in regard to its nature, some of which will be considered hereafter.

We may conveniently separate it into three divisions: Magnetism, which, although formerly ascribed to a special force, is now identified with electricity; Frictional Electricity; and Dynamical Electricity.

468. Natural and Artificial Magnets. - Natural magnets are certain ores of iron, and are generally known under the name of loadstones.

The magnet is so called from the town of Magnesia, in Lydia, where it was first noticed by the Greeks. It is known in chemistry as magnetic oxide of iron. It is now found in considerable quantities in Sweden and Norway, as well as in many other countries.

Artificial magnets are bars of tempered steel, to which the property of the natural magnet has been imparted. The artificial magnet is far more valuable and powerful than the natural magnet, and is generally used in practice.

Steel is a mixture of iron with a small quantity of carbon, and when heated and then plunged into water, it becomes exceedingly hard, and capable of retaining the magnetism that may be imparted to it. Steel magnets are permanent magnets.

Magnets may be made of soft iron or untempered steel, but they do not retain their magnetism when the exciting cause is removed. Such magnets are called temporary magnets.

Artificial magnets for experiment are made of oblong bars, from twelve to fifteen inches in length, as represented in Figs. 332, 333. They are sometimes made in the form of a horse-shoe, as shown in