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eye-glass; and divide these two products by one another, and the quotient is the magnifying power.

James. It is not likely that we should know all these in any instrument we possess.

Tutor. The following then is a method of finding the same thing by experiment. “Observe at what distance you can read any book with the naked eye, and then remove the book to the farthest distance at which you can distinctly read by means of the telescope, and divide the latter by the former.”

Charles. Has not Dr. Herschel a very large reflecting telescope?

Tutor. He has made many, but the tube of the grand telescope is nearly 40 feet long, and 4 feet ten inches in diameter. The concave surface of the great mirror is 48 inches, of polished surface, in diameter, and it magnifies 6000 times. This noble instrument cost the Doctor four years' severe labour : it was finished August 28, 1789, on which day was discovered the sixth satellite of Saturn:

Delighted Herschel, with reflected light, Pursues his radiant journey through the night, Detects new guards, that roll their orbs afar, In lucid ringlets round the Georgian star.

DARWIN.

CONVERSATION XXI.

Of the Microscope --Its Principle-

Of the Single Microscope--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 than about six inches.

Charles. I cannot read a book at a shorter distance than 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 appear, 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 brouglit pear to increase the angle by which it is seen; this is the principle of all mi

croscopes, 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 leps?

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

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