1852.] OF THE ROYAL INSTITUTION. 129 
in the other vertical lines are exhibited in the following table taken 
from the Parthenon, and the other Athenian examples are more or 
less analogous. 
WOlMnSs . ec ws is inwards 1 in 150 
Walls Te CPA or do. 1 iy oO 
Face of Tympanum . . do. 1 in 100 
JAN 01 ee ee) forwards 1 in 80 
Faces of Cornice. . . do 1 in 100 
Faces of Acroteria . . do. 1 in 20 
The horizontal lines when absolutely straight appeared sunk in 
the middle. In the great temple at Pestum, the convexity which 
is intended to obviate this effect is only found in the fronts; and in 
the Propylea at Athens, where the continuity of the stylobate is in- 
terrupted, the curvature is only found in the entablature, the centre 
columns being longer than those towards the angles; thus pointing 
to the influence of the raking lines of the pediment as the origin of 
this optical illusion. The curvature of the stylobate and of the 
flanks followed from the generalization of this idea. This ten- 
dency of the eye to be deceived by the contrast of lines, was illus- 
trated by the following experiments ;: — 
A drawing having been prepared of two columns placed side by 
side, the shafts of which were formed with perfectly straight lines, 
on each side of these, and from top to bottom, the lecturer drew a flat 
circular arc, convex to the outline of one of the columns, and con- 
cave to the other. The column placed between the two concave 
lines instantly seemed attenuated, whilst in the other, about which 
the convex lines were drawn, the effect of a palpable entasis was 
produced. An outline of a pediment had also been provided, con- 
sisting of the horizontal cornice and the two inclined roof lines. 
The cornice was at first alone visible, and appeared perfectly straight, 
as it really was, but when the raking lines of the pediments which 
had been concealed were shown in combination, it appeared to sink 
in the centre. Lastly an elevation of the front of the Parthenon 
drawn to a scale of one-twentieth of the full size, from exact 
measurements obtained at Athens, was exhibited. The line of the 
cornice, really curved, appeared to be straight, owing to the con- 
trast of the inclined lines of the pediment, until a string was strained 
from end to end, when the curvature became evident. 
The lecturer proceeded to describe some of the smaller details of 
the architecture, in which many instances of geometrical science 
are to be found. 
The parabola and hyperbola selected with the parameters at the 
vertex, small in comparison with the length of the arc employed, 
were used in almost all cases where great variety of curvature was ° 
sought. There were even some indications of a knowledge of the 
determination of the radius of curvature in a Conic Section. 
A moulding of circular section was used in the Parthenon only 
in the crowning member or Cymatium. 
It was observed that lines of determinate varied curvature were 
preferable to those arbitrarily drawn, and still more to patched 
