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 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

an inch radius.

part of

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,, and of an inch will each magnify?

James. That is readily done; by dividing 8 inches, the limit of distinct vision, by,, and .

Charles. And to divide a whole number, as 8, by a fraction, as 4, &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 the radii are and will magnify as 8 multiplied by 8, and 8 multiplied by 20; that is, the former will magnify 64 times, the latter 160 times, the diameter of an object.

Tutor. You see then, that the smaller the lens, the greater its magnifying power. Dr. Hooke says, in his work on the microscope, that he has made lenses so small as to be able, not only to distinguish the particles of bodies a million times smaller than a visible point, but even to make those visible of which a million times a million would hardly be equal to the bulk of the smallest grain of sand.

Charles. I wonder how he made them.

Tutor. I will give you his description: he first took a very narrow and thin slip of clear glass, melted it in the flame of a candle or lamp, and drew it out into exceedingly fine threads. The end of one of these threads he melted again in the flame till it run into a very small drop, which, when cool, he fixed in a thin plate of metal, so that the middle of it might be directly over the centre of an extremely small hole made in the plate. Here is a very convenient

single microscope.

Fames. It does not seem, at first sight, so simple as those which you have just now described.

Tutor. A (Fig. 39.) is a circular piece of brass, it may be made of wood, ivory, &c. in the middle of which is a very small hole, in this is

fixed a small lens, the focal distance of which is a D, at that distance is a pair of pliers DE, which may be adjusted by the sliding screw, and opened by means of two little studs. a e; with these any small object may be taken up, and viewed with the eye placed at the other focus of the lens at F, to which it will appear magnified as at I M.

Charles. I see by the joint it is made to fold up.

Tutor. It is; and may be put into a case, and carried about in the pocket, without any incumbrance or inconvenience. Let us now look at a double or compound microscope. James. How many glasses are there in this?

Tutor. There are two; and the