The Elements of Perspective / arranged for the use of schools and intended to be read in connection with the first three books of Euclid

by John Ruskin

Then — is to — as T S is to T D.

That is to say,

a

R is the sight-magnitude of A B.[Footnote 16] ]

Fig. 12.

Therefore, when the position of the point A is fixed in

a

, as in [Fig. 12.], and

a