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SINGLE MICROSCOPE.

this might be, when walking in the garden or the fields! The stamens of a flower, the down of a thistle, or a twig covered with moss, would be found adorned with beauties, of which the unassisted eye has no conception. I have been more particular in describing the single microscope, because it is so cheap, portable, and simple, that any of you may provide yourselves with a lasting amusement, by making it a pocket-companion. Sometimes, in a tortoiseshell case, like this, three lenses, of different magnifying powers, are placed: they may be used separately, or two of the lenses at once, or all three together, according to the degree in which you wish an object to be magnified. Perhaps it is not correct to call this combination a single microscope, as it has several lenses; but it resembles the single microscope in showing the object itself: whereas, in that which is usually called the compound microscope, we see, not the object, but a magnified image of it.

The compound microscope generally contains

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three lenses. By taking this to pieces, I think you will be able to understand how the effect is produced. I will first take the tube in which the glasses are fixed, out of the hollow cylinder in which it slides. This tube is called the body of the instrument. I will speak of it presently. The brass plate, with a hole in the centre, is the stage on which the object to be viewed is placed for inspection; and the use of the hole is to admit of a strong light being thrown upon the object, by reflection from the concave mirror beneath, which is fixed so that it may be turned in any direction, to collect light from a window or candle. In order that this reflected light may fall upon the object, it is laid upon a plate of glass fitted to the hole in the stage; or, if very minute, it may be confined between two small plates of mica, which, being as transparent as glass, much thinner, and less brittle, is preferred for the purpose. In this little ivory box are some mica plates, ready for use; and in this circular brass plate, pierced with holes to receive them, you see a convenient mode

164

EXPLANATION OF THE

of keeping minute specimens ready for ex

amination.

Such are the arrangements made for placing the object and throwing light upon it. We will now examine the tube or body of the microscope, which enables us to inspect the object of our curiosity. On the lower end is screwed an object-glass, which is a small, convex lens, of greater or less magnifying power, as may be required for the present purpose. I have already told you, that the smaller the lens the greater its power of magnifying; but it is better you should see the fact with your own eyes, and then you will be certain of it. In this drawer the object-glasses are arranged and numbered, according to their power. In the first or highest power, the lens you see is very small: the part through which light is transmitted is no bigger than the head of a large pin; as the size of the lens is increased, the power of magnifying diminishes. You see that the fifth, or lowest power, is the largest lens of all. With No. 5, applied close to the eye, I can read a word of three or

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four letters distinctly, and see a space of white paper round the word: but with No. 3, the field of view is so small, that a single letter almost fills the illuminated circle; and with the magnifiers marked 1 and 2, I think you will not be able to distinguish any thing: but take them and try. There is no conviction so clear as that which is produced by our own sensations. When you are satisfied, I will go on.—

We have been using these magnifiers as single microscopes, trusting to the power of a single lens, and looking through it at the object. We find we cannot use the two highest powers to any advantage: the field of view is too small; the light transmitted too feeble. Yet there is no doubt of the superior powers of these small lenses; and if we could obtain more light, and extend the field of view, we might see all the wonders they are capable of exhibiting.

Now the compound microsope was invented just to obviate these difficulties; and I will show you how it acts. It is unnecessary to repeat what has been already said about the

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COMPOUND MICROSCOPE.

concave mirror, placed beneath it, collecting the rays of light, and throwing them more powerfully upon the object. We will now consider what happens, after the rays have passed through the tiny lens, and up this black tube into the body of the instrument.

After passing through the object-glass, the rays of light diverge, and continue receding from each other, till they reach the top of the cylinder. I will unscrew the body of the instrument just above the cylinder, and show you what is placed there to receive them. Here, you see, is a large convex lens, ready to intercept the rays which had diverged too widely to enter the eye of the observer. By passing through this large lens, the rays are made to converge again, till they reach another lens at the top of the tube. I will take it off, and show it to you.

You perceive that each of these lenses has the power of magnifying, but in different degrees as in the small magnifiers, so in these ; the smaller lens magnifies much more than the