Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

Mrs. B. The smallness of the hole, prevents the entrance into the eye, of those parts of every pencil of rays which diverge much; so that, notwithstanding the nearness of the object, those rays from it, which enter the eye, are nearly parallel, and are, therefore, brought to a focus by the humours of the eye.

Caroline. We have a microscope at home, which is a much more complicated instrument than that you have described.

Mrs. B. It is a double microscope, ([fig. 6.]) in which you see, not the object A B, but a magnified image of it, a b. In this microscope, two lenses are employed; the one, L M, for the purpose of magnifying the object, is called the object-glass, the other, N O, acts on the 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 23, 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 imperfection. 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 upon it the whole of the light admitted?

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.