I would like to know if in the following text both options would be correct:
That Space, as our mathematicians have it, is spoken of as having three dimensions, which one may call length, Breadth, and Thickness, and is always definable by reference/referring to three planes, each at right angles to the others.
definable by a reference (needs the article for the noun) . . . .
definable by referring to . . . . (OK)