An Essay on Vision,: Briefly Explaining the Fabric of the Eye, and the Nature of Vision: Intended for the Service of Those Whose Eyes are Weak Or Impaired: Enabling Them to Form an Accurate Idea of the True State of Their Sight, the Means of Preserving It, Together with Proper Rules for Ascertaining when Spectacles are Necessary, and how to Choose Them Without Injuring the Sight