figure 27. Tracing apparatus or glass frame: (a) that designed by
Fried. Christ. Miiller in 1776 (from Meder, p. 547, fig. 253a); (£) that
shown in a sketch by Diirer of 1514 and said to be an exact representa-
tion according to the description by Leonardo (from Meder, p. 546,
fig. 253); (c) an illustration of the use of the apparatus sketched in If
(from a woodcut by Diirer).