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You may test this for yourself in a minute by trying with one eye to look at the same time at a fly on the window pane and a tree in a distant meadow in a straight line beyond the fly you cannot see both together distinctly. When you direct your attention to the tree, your eye adjusts itself for that purpose, and the fly becomes quite indistinct : when you transfer your attention to the fly, you can actually feel that a movement does take place in the eye, which is adjusting itself again to make the fly appear distinct,—and then the tree is blurred and misty.

THE MICROSCOPE.

As a first step towards understanding the microscope, let us see what will be the result if we look at an object (say an arrow), through a double-convex lens. placed beyond the principal focus.

The arrow is to be We shall draw one

pencil of rays from the head of the arrow, and one ray only (to represent a pencil) from the other extremity. Fig. 1 will show what takes place.

Fig. 1.

The rays starting from the head of the arrow meet the lens, are refracted, and come to a focus on the opposite side beyond the principal focus; and the ray from the other extremity of the arrow similarly.

But which is the right way of regarding this figure? Is the eye supposed to be looking from right to left, or from

left to right? Is the small arrow or the large one the object? Just whichever you please. If the small arrow be the object, then the large arrow is the picture formed by the lens, and it is this magnified arrow which the eye sees. On the other hand, if the large arrow be the object, the small arrow is the picture, and in this case your lens makes the object look smaller. You can therefore use a lens in this manner either to magnify or to diminish, but the picture formed by the lens will always be upside down.

Notice that the object and the picture are each beyond the principal focus.

Microscopes are of two kinds, simple and compound.

A simple microscope is nothing more than a lens like that in fig. 1, only it is generally used in a different manner— the object being brought much nearer the lens: in fact, within its principal focus.

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Fig. 2 is an illustration of the use of the simple microscope. The object, mn, is brought within the principal focus F: ma, mb, mc are a pencil of rays which, being refracted by the lens, diverge towards a, b, c, but when produced backwards meet in a focus in M. In like manner a

pencil proceeding from n would be refracted, and meet in a focus in N. Thus a magnified image of the object will be formed between м and N, and it is this magnified image which is seen by the eye (represented in the figure by the letter E).

Now why should the eye see the image M N instead of the object m n? The reason is not hard to see. The rays

a, b, c enter the eye in exactly the same direction as if they had proceeded from M. The eye is therefore deceived, and thinks it sees a large object м N, instead of the small object m n, which it is really inspecting through the lens. In the same manner, you will remember, in a previous lesson, we explained that a shilling could be made to appear where it really is not.

The simplest form of the compound microscope consists of two lenses, the one called the object-glass, used as in fig. I; the other called the eye-glass, used as in fig. 2. Fig. 3 shows how these two lenses are combined to

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Fig. 3.

obtain great magnifying powers: a b is a small object placed beyond the principal focus of the object-glass o. According to what has already been fully explained in fig. 1, there will be a magnified inverted image of the object formed at The lens E, forming the eye-glass, is so placed that this magnified image A B is within its principal focus: consequently the eye, according to what has been already

A B.

explained in fig. 2, sees at a b a magnified image of the image A B. In the figure we have drawn one pencil of rays proceeding from a and coming to a focus at A, which will give you an idea how the first image AB is formed; and then we have drawn a pencil proceeding from в (a point in the image), refracted by the lens E, and brought to a focus at b, in the manner before explained.

These lenses are enclosed in a glass cylinder, which is blackened on the inside, in order to keep out all rays of light proceeding from other objects, so that the object under inspection may alone receive attention.

Therefore any one examining an object through a compound microscope obtains his knowledge of it by means of three successive images :—

1. The object-glass produces a magnified image of the object, between the two lenses.

2. The eye-glass produces a second magnified image of this first magnified image.

3. The lens in the eye causes a third image of this second image to appear on the retina of the eye; and it is from this image on the retina (which is an image of an image of an image of the object), that the mind obtains its knowledge of the object.

THE TELESCOPE.

THE telescope in its simplest form is constructed on the same principles as the compound microscope already described. In the figure we have an arrow ab to represent a distant object. A pencil of rays proceeding from a would be refracted by the object-glass o, and come to a focus at A, and thus a diminished and inverted image of the arrow would be formed at A B.

This image is viewed by the eye through the eye-glass E;

which, as in the

microscope, forms a magnified image a b of the image A B. These lenses are likewise for convenience placed in a brass cylinder.

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What, then, is the difference between a compound microscope and a telescope? There are two chief points of difference.

1. In the microscope the object is near the object-glass; consequently a magnified image is formed by this lens, which image is further magnified by the eye-glass.

In the telescope the object is distant, and therefore a diminished image of the object is formed by the object-glass, which diminished image is magnified by the eye-glass.

Perhaps some one may wonder why we should use an object-glass at all in the telescope if it forms a diminished instead of a magnified image. The answer is, that we must use this object-glass to form an image, in order that we may have this image within our reach. Suppose we are wishing to view the moon. We cannot place the moon, as we can a fly, within the principal focus of our lens; but if we can form an image of the moon, we can place this image where we like. We therefore use the object-glass TO FORM AN IMAGE: in the microscope this image is a magnified one,→ so much the better; in the telescope, however, it cannot be otherwise than a diminished one. We then use the eye-glass TO MAGNIFY THE IMAGE which the other lens has formed.

2. The second chief difference between the compound microscope and the telescope is this, that as the microscope is intended to view a very small object, a very small object-glass is used; but as the telescope is used for large

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