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Binocular Vision.

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mend, however, the concavo-convex lenses represented in O, in fig. 241 for long-sight, and those in R (fig. 242) for short-sight. These are called periscopic glasses, from two words meaning to see round; for, as their shape better enables them to embrace the eyeball, they facilitate vision in all directions; and, as they do not deform objects, they do not fatigue the eye like other glasses.

372. Binocular vision. Although we have two eyes, and when we fix them on the same object, each forms its own image upon the retina, yet we only see one object, just as with two ears we only hear one sound. Various hypotheses have been made to account for single vision with two eyes. Some have considered it as an effect of habit, others assigning to it a physiological cause, have assumed that two points similarly placed on the two retinas correspond to the same nervous filament which, coming from the brain, bifurcates towards each eye. Hence the two impressions simultaneously produced on the two retinas result in a single sensation.

Not only does simultaneous vision with two eyes enable us to see bodies with greater lustre, but it gives us the impression of relief, as the stereoscope well shows.

373. Stereoscope. The stereoscope, so called from two Greek

words which mean perception of solidity, is an ingenious object, which was invented by Sir C. Wheatstone, and modified to, its present form by Sir D. Brewster.

To understand the effect of the stereoscope, let us observe that, when we look at an object with two eyes, each eye does not see it under exactly the same aspect, but under a slightly different perspective. Thus let a small object, such as a dice, be successively viewed with one eye, at a slight distance, without moving the head. If the cube be just in front of the observer, looking at it with his left eye, he will see this face, and a small portion of the left side, the other being concealed (fig. 292); if, on the contrary, he views it with the right eye, he sees the front and a portion of the right side,

the other being hidden (fig. 293). Thus the two images formed on the retina are not quite identical, for each corresponds to a different

m

point of view. It is this difference in the images which gives us the sensation of relief in bodies, and enables us to appreciate their shape and their distance.

This may be confirmed by making two drawings of the same object, taken respectively with the perspective belonging to the right and to the left eye; then, as each eye looks separately at the drawing through prisms or lenses, which make the two drawings coincide, by giving the rays of light the same direction as if they converged from a single object, the impressions produced upon each retina will be the same as if the object itself were viewed. The illusion is in fact so complete, that, however prejudiced we may be, it is impossible not to be deceived, so true are the effects of relief and perspective.

A

C

Fig. 294.

B

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Stereoscope.

381

This is the principle of the stereoscope. Fig. 294 shows the path of the rays of light in the instrument. At A is the drawing of the object seen with the left eye; at B that of the same object seen with the right. The rays from these images fall on two lenses m and n, and take, after having traversed them, the same direction as if they came from the point C; the object represented at A and B appears in relief at this spot.

The lenses m and n must impart exactly the same deviation to the rays, and they should therefore be exactly identical.

Brewster at

tained this result by cutting in two halves a double convex lens, and placing the right half in front of the left eye, and the left half in front of the right eye, as shown in fig. 294.

To produce the sensation of relief, the two dissimilar images at A and B should give from two different points of view so faithful a representation of the same object, that these separate views cannot be taken by the hand. And it is only practicable by means of photography. Fig. 295 represents two photographs of a statuette

of Franklin, taken at a slightly different angle. That of the left represents more of the full face, and must be looked at with the left eye; the other one is more in profile, and must be viewed with the right eye. These two views being placed in the stereoscope disappear for each observer, for then the two sensations special to each eye coalesce, and form a single image as represented in fig. 296, and the original appears so solid, with such perfect relief, that the imagination can with difficulty realise the fact that we are only concerned with a drawing on a plain surface.

374. Natural and artificial magnets.-Natural magnet, or loadstone, is a mineral which has the property of attracting iron and a few other metals, especially nickel and cobalt. This mineral is an oxide of iron, that is, a compound of iron and oxygen like rust, from which it only differs in containing rather less oxygen.

Loadstone has another property, which is not less remarkable, namely, that when it is balanced on a pivot, or suspended to a thread, it points towards a certain direction of the horizon; and by this property this mysterious stone, which is of a dull brown colour, and has no lustre, deserves a place above the most valuable precious stones, for, like a new Ariadne's thread, it guides mariners in darkness, and enables them to steer with the same certainty on sea as on a travelled road.

This loadstone, or magnetic stone, was known to the ancients, who called it Lydian stone, or stone Magnesia; for it was first found near a village of this name in Lydia. And from the town of Magnesia the Greeks derived the name magnes, from which is derived the word magnetism, under which name philosophers understand the whole of the properties which magnets possess. Magnetic iron ore is very abundant in nature; it is met with in the older geological formations, especially in Sweden and Norway, where it is worked as an iron ore, and furnishes the best quality of iron.

Besides natural there are also artificial magnets, so-called from their being produced by art. They are usually made of steel. When steel is tempered, that is, when it is raised to a high temperature, and suddenly cooled by being immersed in cold water,