The field glass or binocular is a combination of two similar telescopes and possesses mechanical adjustments capable of focusing the two telescopes simultaneously or separately, depending upon the type considered. Field glasses are divided into two general classes, viz, the Galilean glasses and the Porro prism glasses. PROPERTIES OF TELESCOPES AND FIELD GLASSES. Telescopes and field glasses have four properties, viz, power, light, field, and definition. These properties are expressed in terms of the corresponding qualities of the unaided eye. Eyes are of very different capabilities. Some people have "short" sight while others have "far" sight. There are normal, excellent, and weak eyes. In the following discussion the capabilities of the normal eye are assumed. For each individual there is a certain distance at which objects may be most distinctly seen. This is called the "visual distance." With shortsighted eyes this distance is from 3 to 6 inches; with normal eyes, from 8 to 14 inches, and with farsighted eyes, from 16 to 28 inches. The capabilities of the normal unassisted eye may therefore be expressed as follows: Power, 1; light, 1; field, 45°; definition, 40´´ to 3´. _Power._--At the "visual distance," all objects seen by the unaided normal eye appear in their natural size. At less than the "visual distance" they appear indistinct, blurred, and imperfectly defined; at greater than the "visual distance" objects are clear and well defined, but diminish in size, the more so as they are farther removed. The ability of a lens to magnify the apparent diameter of an object is termed its power. The power of a lens is defined as the ratio of the diameter of the object as seen through the lens to the diameter as viewed by the unaided eye. The power is also defined as the ratio of the focal distance of the object glass to that of the eyepiece. The power of a field glass can be roughly determined by focusing the instrument on a wall or a range rod, by looking at the object through the instrument with one eye and at the same object directly with the unaided eye. A comparison of the diameter of the two images gives the ratio. The power of a telescope or a field glass can more accurately be measured by means of a dynameter, which is a microscope which can be fitted over the eyepiece end of the instrument, and which magnifies the image. The end of the dynameter next to the eyepiece of the instrument is ruled with a series of lines one-hundredth of an inch apart. On focusing the dynameter, the image of the emerging pencil appears as a sharply defined ring of light with the magnified scale of the dynameter across it. The number of subdivisions covered by the diameter of the ring of light is noted. The diameter of the object glass is similarly measured by means of a pair of dividers and read to the hundredth part of an inch.