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object-glass to be five times as distant as from the eye-glass, and the focal distance of the eye-glass be only the tenth part of
Charles. As five is gained by the distances between the glasses, and 60, 70, and 80, by the eye-glass, the magnifying powers will be as 300, 350, and 400.
James. How is it 60, 70, and 80, are gained by the eye-glass?
Charles. Because the distances of distinct vision are put at 6, 7, and 8 inches, and these are to be divided by the focal distance of the eye-glass, or by it ; but to divide a whole number by a fraction, we must multiply that number by the denominator, or lower figure in the fraction: therefore the power gained by the distance between the two glasses, or 5, must be multiplied by 60, 70, or 80. And the surface of the object will be magnified in proportion to the square
of 300, 350, or 400, that is as 90,000, 122,500, or 160,000.
e now come to the solar 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 4, 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 and reflect them through the tube into the room. The lens ab collects those rays into a focus at n m, where there is another magnifier; here, of VOL. III.
course, the rays cross, and diverge to th white screen on which the image of the ob ject 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 object to the lens, and the farther the screen from it, the greater the 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
nine feet or one hundred and eight inches, or two hundred and sixteen half inches, the diameter of the image will be two hundred and sixteen times larger than the diameter of the object, and this num. ber 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 mi. croscope 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