When the limit of distinct vision is much less than six inches, the individual is said to be short-sighted; when it is much greater than six inches, he is said to be longsighted.

SHORT-SIGHTEDNESS comes from too great convexity of the cornea, or crystalline lens, or both. The effect is to bring the rays to foci before reaching the retina, giving an indistinctness to vision. This defect is remedied by using spectacles with concave lenses, which diverge the rays before falling upon the cornea, and thus enable the media of the eye to bring them to foci upon the retina. If the eyes are unlike, the lenses should be of different power.

LONG-SIGHTEDNESS is a defect just the reverse of shortsightedness. It arises from too great flatness in the cornea, or crystalline lens, so that rays of light are brought to foci behind the retina. This defect is remedied by using spectacles with convex lenses.

Short-sightedness is a defect of youth, and is gradually removed as the individual advances in years; long-sightedness is a defect of advanced age, and once commenced, it gradually increases with years, probably because the organs which secrete the media of the eye become feeble as life advances.

The best form of convex glasses for spectacles is the meniscus, O, Fig. 186, and the best form of concave glasses is the concavo-convex, R, Fig. 187. These glasses are called periscopic, because they permit a wider range of vision than other forms of lenses.

Vision with two Eyes.

342. An image of every object viewed is formed in each eye, yet vision is not double, but single. This is regarded

When is a person short-sighted? When long-sighted? What is the cause of shortsightedness? How is it remedied? What is the cause of long-sightedness? How is it remedied? What are periscopic glasses? (342.) How are we enabled to see clearly with two eyes?

by some as a matter of habit; others refer it to the fact that each nervous filament coming from the brain to the eye is divided into two parts, one going to each eye.

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.

The Stereoscope.

343. The STEREOSCOPE is an apparatus employed to give to flat pictures the appearance of relief; that is, the appearance of having three dimensions.

It was invented by WHEATSTONE and improved by BREWSTER. At the present day it is offered for sale in a great variety of forms, and constitutes an instructive and amusing instrument.

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 dif ferent in the two cases. This will be the more apparent the nearer the body.

B

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.

Fig. 234.

Whence do we derive our notion of relief in bodies? (343.) What is the Stereoscope? By whom invented? Explain the theory and construction of the stereoscope in detail.

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.

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.

Such is the theory of the stereoscope. Fig. 234 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 picture 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 in C.

The lenses, m and n, ought to be perfectly symmetrical, and BREWSTER attained this result 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 daguerreotype or photographic process. The pictures are made by using two cameras inclined to each other in the proper angle.

Fig. 235 represents two stereoscopic pictures of FRANKLIN, taken from a statue. We see the left hand one more in front, the right hand one more in profile. On placing them in the stereoscope we see a single image in relief. This image stands out in relief, presenting all the appearance of the statue from which the pictures are taken.

The best form of the stereoscope is that of DUBOSCQ. The lenses are large, and touch each other, so that they are adapted to eycs which are at any distance apart, which is not the case in the instrument shown in Fig. 234. In that instrument the eyes must be at a certain distance apart, which does not permit the same instrument to be used by both children and adults.

Explain BREWSTER's form.

1. GENERAL

CHAPTER VII.

MAGNETISM.

PROPERTIES OF MAGNETS.

Definition of Magnetism.

344. MAGNETISM, as a science, is that branch of Physics which treats of the properties of magnets, and of their action upon each other.

Magnets.

345. A MAGNET is a body which exercises a particular power of attraction upon iron and a few other metals. Magnets are either natural or artificial.

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. In its natural form it consists of a mixture of two oxides of iron, with a small proportion of quartz and alumina. It is now found in considerable quantities in Sweden and Norway, as well as in many other countries.

The magnet possesses the remarkable power, when freely suspended, of directing itself towards a particular point of the horizon, and it is to this property that its importance is

(344.) What is Magnetism as a science? What is a Magnet? How many kinds of magnets are there? What are natural magnets? Whence the name? What is the constitution of a natural magnet? What remarkable property does the magnet possess?