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the radii are į and it will magnify as 8 multiplied by 8, and 8 multiplied by 20; that is, the former will magnify sixty-four 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 visi ble 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.
James. 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 D Е, 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 1 m.
Charles. I see by the joint it is made to
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 construction of it may be seen by this figure ; cd (Fig. 40.) is called the object-glass, and e f the eye-glass. The small object a b is placed a little farther from the glass cd than its principal focus, so that the pencils of rays flowing from the different points of the object, and passing through the glass, may be made to converge and unite in as many points between & and h, where the image of the object will be formed. This image is viewed by the eye-glass e f, which is so placed that the image g h may be in the focus, and the eye at about an equal distance on the other side, the rays of each pencil will be parallel after going out of the eye-glass, as at e and f, till they come to the eye at k, by the humours of which they will be converged and collected into points on the retina, and form the large inverted image A B.
Charles. Pray, sir, how do you calculate the magnifying power of this microscope?
Tutor. There are two proportions which, when found, are to be multiplied into one another : (1.) As the distance of the image from the object-glass is greater than its distance from the eye-glass; and, (2) as the distance from the object is less than the limit of distinct vision. *
Example. If the distance of the image from the object-glass be four times greater
Dr. Vince gives the following rule for finding the linear magnifying power of a compound microscope : “ It is equal to the least distance of distinct vision, multiplied by the distance of the image from the object-glass, divided by the distance of the ob. ject from the object-glass, multiplied by the focal length of the eye-glass."
than from the eye-glass, the magnifying power of four is gained ; and if the focal distance of the eye-glass be one inch, and the distance of distinct vision be considered at seven inches, the magnifying power of seven is gained, and 7 multiplied by 4 gives 28 ; that is, the diameter of the object will be magnified twenty-eight times, and the surface will be magnified 784 times.
James. Do you mean that an object will, through such a microscope, appear 784 times larger than by the naked eye e ?
Tutor. Yes, I do; provided the limit of distinct vision be seven inches ; but some persons who are short-sighted, can see as distinctly at five or four inches, as another can at seven or eight: to the former the object will not appear so large as to the latter.
Ex. 2. What will a microscope of this kind magnify to three different persons, whose eyes are so formed as to see distinctly at the distance of 6, 7, and 8 inches by the naked eye; supposing the image of the