The capability of the instrument, however, is thereby much limited; great powers give plain images only with relatively long tubes; glasses must be held the steadier the more they magnify; and with increasing power all vibrations become more troublesome and render minute observations very difficult or impossible. The additional lenses in terrestrial telescopes somewhat decrease power and affect also light and definition. It is clear therefore that expectations of achieving great power should not be entertained, the function of field glasses being to bring out and define objects which to the naked eye appear indistinct and doubtful. The distinctness with which anything can be seen through the telescope depends, primarily, upon the number of straight lines of light which are collected by it from every point of the object. Telescopes, the object glasses being equal in size, diminish light as a general rule in proportion as their magnifying power is great. The most powerful glasses are therefore to be used for minute observations on the clearest days or when there is a strong light upon the observed object. When the light is fading or there is a little light upon the observed object the clearer view will be had with glasses of large field and low magnifying power. FIELD GLASSES AND TELESCOPES ISSUED BY THE SIGNAL CORPS. The Signal Corps issues four standard field glasses, viz, Type A, Type B, Type C, Type D. Field glasses issued by the Signal Corps are not supplied for the personal use of an officer and will not be used in lieu of the officer's personal field glass prescribed by paragraph 97, General Orders, 169, War Department, 1907 (Par. 1, G. O. 16, War Dept., 1910). Under paragraph 1582, Army Regulations, as amended by paragraph I, General Orders, No. 207, War Department, October 16, 1909, the Signal Corps will sell field glasses to officers of the army for their personal use. Application for the purchase of field glasses should be addressed to the Chief Signal Officer of the Army, Washington, D. C., inclosing post-office money order or check on the Treasurer or Assistant Treasurer of the United States for the amount, payable to the Disbursing Officer, Signal Corps, and Signal Corps Form No. 240 accomplished in duplicate. The Government does not pay transportation charges for the shipment of articles sold to officers. Field glasses are sent from the Signal Corps General Supply Depot, Fort Wood, New York Harbor, by express, charges collect, unless purchase request is accompanied by funds so that field glasses may be sent by registered mail. Forwarding by registered mail is somewhat cheaper than by express, and the amount of postage required is 40 cents for Type D glass, 46 cents for Types A and B, and 74 cents for Type C. Express charges depend upon the distance from New York.