CHAPTER IV.--_The field message._ Definition 64 The blank form 64 Writing the message 66 Instructions to operators: Use of message blank 66 Duties of sending operators 66 Order of transmission 66 Duties of receiving operators 67 Communications confidential 67 Checking the message 67
CHAPTER V.--_The signal station._ Location of stations: General considerations 68 Backgrounds 70 Azimuth of stations 71 Altitude 71 Determination of background color 72 Choice of apparatus 73 Miscellaneous considerations 73 Intervisibility table 74 Finding a station 75 Operation of stations: Personnel 76 Calls and personal signals 78 Opening communication 79 Commencing the message 80 Sending and receiving 80 Breaking 80 Discontinuance of transmission 81 Acknowledgment of receipt 81 Station records 81 Formation of signals 82 Repeating the message 83 Signal practice 83
CHAPTER VI.--_Codes and ciphers._ Codes in use 84 Employment of codes 84 Cipher code 85 The War Department Code 86 Cipher code in field work 87 Field ciphers: Description and use 87 Forms of field cipher 88 Inversions 88 Concealment of terminations 88 Cipher apparatus: The cipher disk 89 The mathematical cipher 93 The route cipher 94 Cipher detection: Employment of cipher disk 96
CHAPTER VII.--_Field glasses and telescopes._ Reflection 98 Refraction 98 Lenses 98 Focus 99 Optical center 99 Image 99 Conjugate foci 99 Law of foci 100 Formation of image 101 Spherical aberration 102 Chromatic aberration 102 Telescopes 104 Galilean field glasses and telescopes 106 Porro prism field glasses and telescopes 106 Field glasses 108 Properties of telescopes and field glasses 109 Power 109 Light 111 Field 114 Definition 115 Field glasses and telescopes issued by the Signal Corps 119 Type A 121 Type B 124 Type C 125 Type D 125 Field-glass specifications 126
While, in consequence of the development of electrical invention and improvement, visual signaling will be less frequently resorted to in future than heretofore in the service of field lines of information, it should be appreciated that the necessity for an adequate supply of apparatus of this kind, and the need for skilled manipulators to operate it, has in no wise diminished. The great celerity with which electric signals can be exchanged and their usual entire independence of local conditions has placed systems of this class foremost among the signaling methods of the world. There is scarcely any commercial industry whose successful existence does not vitally depend upon some one, perhaps several systems of signaling, and improvements of old and inventions of new signal devices are continually necessary to meet the requisite needs demanded by the progress of art and science. Railways are probably the greatest of all commercial users of signals. With them the great mass of intelligence is transmitted by the electric telegraph and telephone, but the flag, the semaphore, the signal light, and many other contrivances furnish indispensable visual adjuncts. Visual signaling is and always will be a most valuable means of transmitting information in peace and war, and it is not to be imagined that it will ever be supplanted in its particular function by the introduction of other methods. Occasions will frequently occur in the field when no other means will be practicable, and then, if not before, will the value of the system be fully emphasized.