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.