[A] In these directions "1-1/2 _in._ power" means a lens having a focus of 1-1/2 inches; "1/2 _in._ power" means a lens or combination of lenses having a focus of 1/2 inch; and so on. All the different powers mentioned in the directions may be obtained in the small-sized 3-lens, bellows form magnifier, either by using the lenses singly or combined in different ways. The magnifying power of any single lens or simple combination is easily found by dividing 10, by the focus in inches. Thus the magnifying power of a 1/2 _in._ lens is found in this way: 10/1/2 = 10x2/1 20. The lens magnifies therefore 20 diameters _i. e._ makes an object appear twenty times as long and twenty times as broad as it is. TRANSCRIBER'S NOTES: Text in italics is surrounded with underscores: _italics_. Inconsistencies in hyphenation have been retained from the original. Obvious typographical errors have been corrected as follows: Page 8: hundreth changed to hundredth Page 11: iustrument changed to instrument Page 15: diferent changed to different Page 17: he changed to the Page 20: wil changed to will Page 44: or changed to of Page 72: staightened changed to straightened Page 86: DIFFRENT changed to DIFFERENT Page 125: fouths changed to fourths