Scouting, Patrolling and Sniping - FM 21-75 War Department Field Manual

13. NIGHT VISION, a. General. The human eye adapts itself for seeing in the dark by enlarging the pupil in order to let in more light. Night vision decreases with fatigue and is sometimes affected by vitamin deficiency, b. Preparations. Before leaving on a night mission the scout should prepare his eyes by remaining in complete darkness for about an hour if possible. If he cannot stay in darkness he should keep out of the lights around him as long as possible and avoid looking straight at them, use red goggles or red light, or keep one eye covered- If the eyes are exposed to light, even though red, full night vision is retarded or lost. (See par. 37b.) c. Training. Practice is necessary in order to acquire the ability to use and care for the eyes at night Exercises are contained in chapter 7. 14. APPEARANCE OF OBJECTS. Darkness not only makes it difficult or impossible to see objects, it also changes their appearance and apparent size, and details are blotted out. A tree seen against the night sky looks much smaller than it does in the daytime because the twigs at the tips of the branches cannot be seen at night. For die same reason, an airplane caught in the beam of a searchlight looks larger than the same plane seen as a black mass against a dimly lighted sky. Under ordinary conditions, a match can be seen from a plane or from the ground for several miles. Under ideal conditions of darkness and atmosphere, a candle is said to be visible for ten miles. The scout has to train himself to identify objects by black outlines at night. He cannot depend upon details that are visible in daylight for identification. Night glasses make it possible to see objects or parts of objects that would be otherwise too small to be seen at all, and help to identify objects already spotted. 15. SOUNDS. A scout must depend largely upon his ears to obtain information of the enemy at night. Similarly, the enemy may detect him if he makes any noise. The scout must stop frequently to listen, removing his helmet (if worn) to eliminate unnatural or distorted sounds. The ability to listen for long periods in perfect silence must be cultivated by constant practice. Sound travels approximately 370 yards a second. When a scout sees a flash, he can often estimate the range to the weapon. He counts rapidly during the time interval between the flash and the hearing of the report. If he counts rapidly to three, for example, the range is approximately 300 yards. The correct cadence must be determined by actual practice at known distances. Sounds are transmitted a greater distance in wet weather and at night than in dry weather and in the daytime. If the scout holds his ear close to the ground, he can hear such sounds as the walking of persons and the noise of vehicles much better. The scout must be trained to identify and estimate the direction and distance to various common noises made at night by troops in the field. 16. SMELLS. The scout's sense of smell may warn him of enemy fires, cooking, picket lines, motor parks, gasoline and oil engines, bodies of water, and the presence of troops generally. Types of shells may also be identified by the characteristic odors of their bursts. 17. TOUCH. The scout must be able to feel and to recognize objects in the dark. He must be able to adjust and operate his equipment quietly by the sense of touch alone. 18. GENERAL. Although total darkness provides concealment, there will be many bright nights when the scout must observe the same principles of concealment as in the daytime. (See par.

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