INTRODUCTION.

instances, that have been given: I will offer one of afamiliar kind, which may be a hint for their observinga thousand more. How gradually does the eye growreconciled even to a disagreeable dress, as it becomesmore and more the fashion, and how soon return to itsdistike of it, when it is left off, and a new one hastaken possestion of the mind ? —so vague is taste, whenit has no solid principles for its foundation.!

Notwithstanding I have told you my design ofconsidering minutely the variety of lines, which serve toraise the ideas of bodies in the mind, and which areundoubtedly to be consider'd as drawn on the sur-faces only of solid or opake bodies: yet the endea-vouring to conceive, as accurate an idea as is possible,of the inside of those surfaces, if I may be allow’d theexpression, will be a great assistance to us in the pur-suance of our present enquiry.

In order to my being well understood, let every objectunder our consideration, be imagined to have its in-ward contents fcoop’d out so nicely, as to have nothingof it left but a thin shell, exactly corresponding both inits inner and outer surface, to the shape of the objectitself: and let us likewise suppose this thin shell to bemade up of very sine threads, closely connected toge-ther, and equally perceptible, whether the eye is sup-posed to observe them from without, or within ; andwe shall find the ideas of the two surfaces of this shellwill naturally coincide. The very word, shell, makesus seem to see both surfaces alike. But