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which are the effects of reflection, the concave mirror is substituted for the convex lens. (Fig. 36.) represents the large tube, and t t the small tube of the telescope, at one end of which is D F, a concave mirror, with a hole in the middle at P, the principal focus of which is at IK; opposite to the hole P, is a mirror L, concave towards the great one; it is fixed on a strong wire м, and may, by means of a long screw on the outside of the tube, be made to move backwards or forwards. A B is a remote object from which rays will flow to the great mirror D F.

:

James. And I see you have taken only two rays of a pencil from the top, and two from the bottom.

Tutor. And in order to trace the progress of the reflections and refractions, the apper ones are represented by full lines, the lower ones by dotted lines. Now the rays at c and E falling upon the mirror at D and F, are reflected, and form an inverted image at m.

Charles. Is there any thing there to receive the image?

Tutor. No: and therefore they go on towards the reflector L, the rays from different parts of the object crossing one another a little before they reach L.

James. Does not the hole at P tend to distort the image?

Tutor. Not at all; the only defect is, that

there is less light. From the mirror L the rays are reflected nearly parallel through P, there they have to pass the plano-convex lens r, which causes them to converge at a b, and the image is now painted in the small tube near the eye.

Charles. What is the other plano-convex lens s for?

Tutor. Having by means of the lens R, and the two concave mirrors, brought the image of the object so nigh as at a b, we only want to magnify the image.

James. This, I see, is done by the lens s. Tutor. It is, and will appear as large as cd, that is, the image is seen under the angle cfd. Charles. How do you estimate the magnifying power of the reflecting telescope?

Tutor. The rule is this: "Multiply the focal distance of the large mirror by the distance of the small mirror from the image m: then multiply the focal distance of the small mirror by the focal distance of the 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 dişVOL. III.-K

tinctly 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 four 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 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.

James. 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 readingglass 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 part of an inch radius.

James. And in a double convex the focal distance is always equal to the radius of convexity. Tutor. Then tell me how much lenses of 4, , and of an inch will each magnify ?