construction of it may be seen by this figure; c d (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 c d 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 g 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 objectglass 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*.

* 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,

Example. If the distance of the image from the object-glass be 4 times greater than from the eyeglass, the magnifying power of 4 is gained; and if the focal distance of the eye-glass be one inch, and the distance of distinct vision be considered at 7 inches, the magnifying power of 7 is gained, and 7 X 4 gives 28; that is, the diameter of the object will be magnified 28 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?

divided by the distance of the object from the object-glass, multiplied by the focal length of the eye-glass."

Tutor. Yes, I do; provided the limit of distinct vision be 7 inches; but some persons who are shortsighted, can see as distinctly at 5 or 4 inches, as another can at 7 or 8: 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 object-glass to be five times as distant as from the eye-glass, and the focal distance of the eye glass be only the tenth part of an inch?

Charles. As five is gained by the distances between the glasses, and 60, 70, and 80, by the eye-glass, the

magnifying powers will be as 300,

350, and 400.

James. How is it 60, 70, and SO, are gained by the eye-glass?

Charles. Because the distances of distinct vision are put at 6, 7, and 8 inches, and these are to be divided by the focal distance of the eye-glass, or by but to divide a whole number by a fraction, we must multiply that number by the denominator, or lower figure in the fraction: therefore the power gained by the distance between the two glasses, or 5, must be multiplied by 60, 70, or 80. And the surface of the object will be magnified in proportion to the square of 300, 350, or 400, that is, as 90,000, 122,500, or 160,000.

Tutor. We now come to the solar