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microscope, which is by far the most entertaining of them all, because the image is much larger, and being thrown on a sheet, or other white surface, may be viewed 'by many spectators at the same time, without any fatigue to the eye. Here is one fixed in the window-shutter, but I can explain its construction best by a figure.
James. There is a looking-glass on the outside of the window.
Tutor. Yes, the solar microscope consists (Plate vi: Fig. 42.) of a looking-glass so without, the lens a b in the shutter d U, and the lens n m within the dark room. These three parts are united to, and in a brass tube. The looking-glass can be turned by the adjusting screw, so as to receive the incident rays of the sun s s s, and
reflect them through the tube into the room. The lens a b collects those rays into a focus at n m, where there is another magnifier; here, of course, the rays cross, and diverge to the white screen on which the image of the object will be painted.
Charles. I see the object is placed a little behind the focus.
Tutor. If it were in the focus it would be burnt to pieces immediately. The magnifying power of this instrument depends on the distance of the sheet or white screen; perhaps about 10 feet is as good a distance as any. You perceive that the size of the image is to that of the object as the distance of the former from the lens n m, is to that of the latter.
James. Then the nearer the ob
joct to the lens, and the farther the screen from it, the greater power of this microscope.
Tutor. You are right, and if the object be only half an inch from the lens, and the screen nine feet, the image will be 46,656 times larger than the object: do you understand this?
Charles. Yes, the object being only half an inch from the lens, and the image 9 feet or 108 inches, or 216 half inches, the diameter of the image will be 216 times larger than the diameter of the object, and this number multiplied into itself will give 46,656.
Tutor. This instrument is calculated only to exhibit transparent objects, or such as the light can pass through in part. For opaque objects, a different microscope is used: and, indeed, there are an indefinite number of microscopes, and of them all, we may say, though in different degrees :
The artificial convex will reveal
Of the Camera Obscura, Magic Lan,
thorn, and Multiplying Glass.
TUTOR. We shall now treat upon some miscellaneous subjects; of which the first shall be the Camera Obscura.
Charles. "What is a camera obscura?
Tutor. The meaning of the term is a darkened chamber: the construction of it is very simple, and will be understood in a moment by you, who