The principles

Since any opaque object absorbs light, it produces a shadow in the space behind it.
If the source of light is a point, an opaque surface cuts off all light striking it, producing a shadow of uniform density.
An example is the casting of hand shadows on a wall.

If the source of light is larger than a point, the shadow varies in intensity, creating the umbra and penumbra.
The former is that portion from which all rays of light are obscured, while the penumbra is the lighter part, not entirely hidden from the observer.

Spotlights, hand torches, and so on, are points of light. The latter of the two shadows is the more frequently encountered. Thus, in Ninjitsu, we strive to remain in the deepest shadow, the umbra, as this offers the best concealment.

The rule of Kagashi-no-jitsu states that the eye sees movement first, silhouette second, and color third.
Dark adaptation means allowing the eyes to become accustomed to low levels of light.
Approximately thirty minutes are required for the rod cells of the eye to produce sufficient visual purple to enable the eye to distinguish objects in dim light.

Off Center vision is a technique of focusing attention on an object without looking directly at it.
When an object is looked at directly, the image is formed on the cone region of the eye.
This area is not sensitive at night.
When the eye looks five to ten degrees above, below, right, or left of the object, the image falls on the rod cells, making it visible in dim light.

Scanning is a method of using this off-center vision to observe an object or area.During night observation, the visual purple of the rod cells bleaches out in five to ten seconds and the image fades.
As this occurs, you must shift the eyes slightly so that fresh rod cells are used.

Move the eyes in short, irregular intervals over the object, but do not look directly at it.

Pause a few seconds at each point of observation because your eyes normally are used where there is sufficient light to create sharp outlines and bright colors.
In darkness, objects are faint, have no distinct outline, and little or no color. To move in darkness, you must believe what you see.
Only practice can achieve this.At night, if the enemy can be seen, keep the fire (light) between yourself and the enemy.

Remember, the enemy is looking from and area of light (in which his pupils have constricted) into an area of darkness, where insufficient light exists to display an image on the cone region of the retina.
In daylight, keep the fire and the door on your right, and keep the left side clear.

Moving in shadows requires that a path be selected from one place of concealment to another, crossing any exposed areas quickly and quietly Standing in darkness requires great patience and controlled breathing.
The best place inside a room is the nearest corner behind the door. Select a shadow to be used and advance silently to it.
Assume a posture which conforms to the shape of the shadow and remain within it.
Practice shallow breathing.

To become invisible, Ninjitsu employs the Nine Steps of Kuji Ashi.
This is consistent with the concept of Shugendo, the mountain asceticism of feudal Japan, in which Kuji (nine) is the most important number.
Nine is the number of completion in numerology.

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