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day was discovered the sixth satellite of Saturn :
Delighted Herschel, with reflected light.
of the Microscope-Its Principle of the Single Mi.
croscope-Of the Compound Microscope of the Solar Microscope
TUTOR. We are now to describe the microscope, which is an instrument for viewing very small objects. You know that, in general, persons who have good sight cannot distinctly view an object at a nearer distance that about six inches.
Charles. I cannot read a book at a shorter distance then this; but if I look through a small hole made with a pin or needle in a sheet of brown paper, I can read at a very small distance indeed.
Tutor. You mean, that the letters ap- 2 pear, in that case, very much magnified, the reason of which is, that you are able to see at a much shorter distance in this way than you can without the intervention of the paper. Whatever instrument, or contrivance, can render minute objects visible and distinct, is properly a microscope.
James. If I look through the hole in the paper at the distance of five or six inches from the print, it is not magnified.
Tutor. The object must be brought near to increase the angle by which it is seen ; this is the principle of all microscopes, from the single lens to the most compound instrument. A (Plate vi. Fig. 37.) is an object not clearly visible at a less distance than A B; but if the same object be placed in the focus c (Fig. 38.) of the lens D, the rays which proceed from it will become parallel, by passing through the said lens, and therefore the object is distinctly visible to the eye at E,
placed any where before the lens. There are three distinctions in microscopes; the single, the compound, and the solar.
Charles. Does the single microscope consist only of a lens ?
Tutor. By means of a lens a great number of rays proceeding from a point are united in the same sensible point, and as each ray carries with it the image of the point from whence it proceeded, all the
rays united must form an image of the object.
Jumes. Is the image brighter in proportion as there are more rays united ?
Tutor. Certainly : and it is more distinct in proportion as their natural order is preserved. In other words, a single microscope or lens removes the confusion that accompanies objects when seen very near by the naked eye; and it magnifies the diameter of the object, in proportion as the focal distance is less than the limit of distinct vision, which we may reckon from about six to eight inches.
Charles. If the focal distance of a reading glass be four inches, does it magnify the diameter of each letter only twice?
Tutor. Exactly so: but the lenses used in microscopes are often not more than or or even zo part
inch radius. And in a double convex the focal distance is always equal to the radius of convexity.
Tutor. Then tell me how much lenses of }, j, and ab of an inch will each magnify?
-James. That is readily done; by dividing 8 inches, the limit of distinct vision, by ,, and o's.
Charles. And to divide a whole number, as 8, by a fraction, as , &c. is to multiply the said number by the denominator of the fraction : of course, 8 multiplied by 4, gives 32 ; that is, the lens, whose radius is a of an inch, magnifies the diameter of the object 32 times.
James. Therefore the lenses of which