This section is from the book "The Lady's Assistant: Family Physician", by P. Davey and B. Law.
The mod common cause of squinting, is an inequality of the strength of the eyes. To prove this, place the person near a window, and cause him to look at a small object, a pen for instance; you will readily perceive which eye is directed towards it; then cover that eye with your hand, and the person will be surprized to find that he has quite loft the pen; which is a demonstration, that he fees but with one eye : therefore the best method of cure, is to exercise the weak eye by covering the other now and then, and making use of the weak eye only. This method will generally succeed when the squinting is curable.