principle of the single microscope, and is called the eye-glass.

There is another kind of microscope, called the solar microscope, which is the most wonderful from its great magnifying power: in this we also view, an image formed by a lens, not the object itself. As the sun shines, I can show you the effect of this microscope: but for this purpose, we must close the shutters, and admit only a small portion of light, through the hole in the window-shutter, which we used for the camera obscura. We shall now place the object A B, (plate XXIII. fig. 1.) which is a small insect, before the lens C D, and nearly at its focus; the image E F, will then be represented on the opposite wall in the same manner as the landscape was in the camera obscura; with this difference, that it will be magnified, instead of being diminished. I shall leave you to account for this, by examining the figure.

Emily. I see it at once. The image E F is magnified, because it is farther from the lens, than the object A B; while the representation of the landscape was diminished, because it was nearer the lens, than the landscape was. A lens, then, answers the purpose equally well, either for magnifying or diminishing objects ?

Mrs. B. Yes: if you wish to magnify the image, you place the object near the focus of the lens; if you wish to produce a diminished image, you place the object at a distance from the lens, in order that the image may be formed in, or near the focus.

Caroline. The magnifying power of this microscope, is prodigious; but the indistinctness of the image for

want of light, is a great imperfeetion. Would it not be clearer, if the opening in the shutter were enlarged, so as to admit more light.

Mrs. B. If the whole of the light admitted does not fall upon the object, the effect will only be to make the room lighter, and the image consequently less distinct.

Emily. But could you not by means of another lens bring a large pencil of rays to a focus on the object, and thus concentrate the whole of the light admitted upon it ?

Mrs. B. Very well. We shall enlarge the opening, and place the lens X Y (fig. 2.) in it, to converge the rays to a focus on the object A B. There is but one thing more wanting to complete the solar microscope, which I shall leave to Caroline's sagacity to discover.

Caroline. Our microscope has a small mirror attached to it, upon a moveable joint, which can be so adjusted as to receive the sun's rays, and reflect them upon the object; if a similar mirror were placed to reflect light upon the lens, would it not be a means of illuminating the object more perfectly.

Mrs. B. You are quite right. P Q (fig. 2.) is a small mirror placed on the outside of the window shutter, which receives the incident rays S S, and reflects them on the lens X Y. Now that we have completed the apparatus let us examine the mites on this piece of cheese, which I place near the focus of the lens.

Caroline. Oh, how much more distinct the image now is, and how wonderfully magnified; the mites on the cheese look like a drove of pigs scrambling over rocks ?

Emily. I never saw any thing so curious. Now, an

immense piece of cheese has fallen : one would imagine it an earthquake: some of the poor mites must have been crushed; how fast they run,-they absolutely seem to gallop.

But this microscope can be used only for transparent objects; as the light must pass through them to form the image on the wall ?

Mrs. B. Very minute objects, such as are viewed in a microscope, are generally transparent; but when opaque objects are to be exhibited, a mirror M N (fig. 3.) is used to reflect the light on the side of the object next the wall: the image is then formed by light reflected from the object, instead of being transmitted through it.

Emily. Pray is not a magic lanthorn constructed on the same principles ?

Mrs. B. Yes; with this difference that the light is supplied by a lamp, instead of the sun.

The microscope is an excellent invention, to enable us to see and distinguish objects, which are too small to be visible to the naked eye. But there are objects which, though not really small, appear so to us, from their distance; to these we cannot apply the same remedy; for when a house is so far distant, as to be seen under the same angle as a mite which is close to us, the effect produced on the retina is the same : the angle it subtends is not large enough for it to form a distinct image on the retina.

Emily. Since it is impossible, in this case, to approach the object to the eye, cannot we by means of a lens bring an image of it nearer to us?

Mrs. B. Yes; but then the object being very distant