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from an anecdote which was related to me by a young person who witnessed the appearance.

It happened that she got up very early one summer morning, before any one else was stirring, and passed through the small porch or entry of the house; the outer door fronted the east, and through the key-hole streamed a bright ray, which fell upon the whitewashed wall opposite, perhaps about three feet distant. There, to her great delight and wonder, she saw pourtrayed an exact picture of the farm-yard, with the poultry, pigeons, and other animals, in their natural colours; but the magic picture was inverted; the creatures appeared to be walking or flying upside down. The young person who saw this moving scene, was satisfied with admiring so novel a sight, and I dare say would not have thought of any way of setting the images upright; neither would she have invented the means of reproducing such pictures at pleasure. I will now show you how Baptista Porta contrived to do this.

The simplest kind of camera-obscura, and

174 SIMPLEST FORM OF THE CAMERA,

therefore, probably, the first that was made, is formed by placing a convex lens in a hole, cut purposely to receive it, in a window-shutter. You already know that rays of light after passing through such a lens, will converge; therefore, on a white wall or a sheet of paper, placed in the focus of the lens, there will be a beautiful inverted picture of all the objects on the outside, especially if the sun happens to be shining brightly upon them.

In order to see this picture, the observer must however be shut up in the darkened room; and even there he can see only such objects as are before the window: they will indeed be more vivid and distinct than if no lens were employed, but they will be inverted. To obviate this imperfection, recourse has been had to various expedients. I am not sure which of them was adopted by the original inventor; we will now confine ourselves to those which are seen in the instrument before us.

First, you will observe the convex lens, which is fitted into a frame, made to slide backward

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and forward in the end of the box: by means of the slide, the focus may be adjusted, as it is in a telescope or microscope. You will perhaps inquire, "How are we to see the images when they are inclosed in that box?" I will show you. By lifting up this mahogany flap, we obtain a view of what is going on in the interior of our little, darkened chamber. After the Lecture is over, we will take a view of some of the present company, and you will then see the miniature portraits reflected, not where you would naturally look for them, inverted on the end of the box opposite to the lens, as they were on the wall of the farm-house porch that I mentioned, but in their natural position, on this plate of ground glass; and I dare say you wonder how that reflection can proceed from a lens fixed perpendicularly in the other end of the box. We will remove the horizontal, or level plate of glass, and beneath it we shall discover the mystery.

Here, you perceive, is a plain mirror, or common looking-glass, reclining backward from the

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large one, and the image of the object to be viewed being formed between these two lenses, the rays proceeding from it are made again to converge, on passing through the upper lens or eye-glass, in the focus of which, a distinct and greatly magnified image of the object is seen.

I hope this drawing will make the affair still plainer. The first figure is a microscope, like this we have been examining: the second is a diagram, in which slanting lines show the course of the rays, and the refractions they sustain in passing through the different lenses.

The arrow a b, represents any small object, which you may suppose to be laid on the stage of the microscope, as at F. C is the objectglass, such as you have seen screwed on the instrument at the part marked O in the drawing. The rays proceeding from the object, on entering his small lens, are converged. They cross one another, and pass out of the lens diverging: if hey met with no interruption, these rays would continue to rise up through the body of the microscope, and would form, in the upper part

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