If, therefore, 45,240 tenths inches correspond to 1,296,000′′, then 1 tenth inch equals 1,296,000 divided by 45,240, or 28.6′′. The definition is therefore 28.6′′, or practically half a minute.
The capabilities of glasses, including telescopes, in a general way, lie between the following limits:
(1) Power between 2 and 1,000.
(2) Light may be 0.01 to 200 times that of the unaided eye.
(3) Field measures in most favorable case, 10°; in the most unfavorable, .01°.
(4) Definition varies between 40′′ and 0.1′′.
Thus, as a maximum, an object may be seen by means of a telescope, magnified 1,000 times, 200 times brighter and 400 times sharper than with the naked eye.
If these advantages could be fully utilized for military purposes the use of glasses would be extraordinary, a power of 1,000 practically effecting the same purpose as the approach of the observed object to one-thousandth of the distance. A hostile command 10 miles distant could be seen theoretically as well as if they were only 53 feet away, and the slightest movement of each single man would become visible. Of course no such wonderful effect is physically practicable, and the limiting conditions increase greatly in proportion as either one or the other of the qualities, power, field, etc., is especially sought.
While astronomers require only that the telescope be made as capable and perfect as possible in an optical point of view, making all other conditions subordinate to this one, the military, to whom the glass is simply an accessory, make other conditions of the first importance. The glass must have suitable form, small volume, little weight, and that it may be used without support, mounted or dismounted, and the image must appear as looked at by the naked eye—that is, not inverted.