Note:  Do not rely on this information. It is very old.


Deviation. A mariner's compass should lie in the magnetic meridian, but neighbouring masses of iron and steel will cause some deviation from this position. It is often difficult to avoid this, though the consequences of an unknown deviation of the needle may be very serious. If possible, the compass should be placed far from all such magnetic matter as steel or iron; and if this cannot be done, suitable bars should be so adjusted as to neutralise their deviating influence.

In Optics, deviation means the change in direction of rays or waves of light by reflection or refraction. If, for example, light strikes a mirror at an angle of 45°, a deviation of 90° is produced by the reflecting surface. By reason of the deviation of light in passing obliquely through the atmosphere we are sometimes able to see the moon before it is actually above the horizon.