Note 2 — Chapter I · THE POCKET LENS, THE DISSECTING MICROSCOPE, AND SOME SIMPLE APPLIANCES
Reading edition
The power of magnification of a lens is the ratio of its focal distance to 10 inches. Thus a lens of 1 inch focus (or focal distance) magnifies 10 times (written × 10, or ten diameters); one of ½ in. focal distance, 20 times, and so on.