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What is primary and secondary deviation Squint?

Primary deviation is the deviation of the "lazy" or paretic eye, when the "good eye" or the non-paretic eye fixes on an object. 

Secondary deviation is the deviation of the “ good "or non- paretic eye, when the "lazy" eye or the"paretic eye" fixes on an object.

Secondary deviation is always greater than primary deviation in noncomitant strabismus because of Hering's law of equal innervation.

When the left eye is covered right eye has to move more to fixate.same innervation go to the left eye also.