Many optical illusions are found in architecture and, strangelyenough, many of these were recognized long before painting developedbeyond its primitive stages. The architecture of classic Greecedisplays a highly developed knowledge of many geometrical opticalillusions and the architects of those far-off centuries carefullyworked out details for counteracting them. Drawings reveal many opticalillusions to the architect, but many are not predicted by them. Theever-changing relations of lines and forms in architecture as we varyour viewpoint introduce many optical illusions which may appear anddisappear. Any view of a group of buildings or of the components of asingle building will exhibit some optical illusions. We never see inthe reality the same relations of lines, forms, colors, andbrightnesses as indicated by the drawings or blue-prints. Perhaps thisis one of the best reasons for justifying the construction of expensivemodels of our more pretentious structures.No detailedaccount of the many architectural optical illusions will be attempted,for it is easy for the reader to see many of the possibilitiessuggested by preceding chapters. However, a few will be touched upon toreveal the magnitude of the illusory effect and to aid the observer inlooking for or recognizing them, or purely for historical interest. Inarchitecture the eye cannot be wholly satisfied by such tools as thelevel, the square, and the plumb-line. The eye is satisfied only whenthe appearance is satisfactory. For the purpose of showing theextent of certain architectural optical illusions, the compensatorymeasures applied by the Greeks are excellent examples. These alsoreveal the remarkable application of science to architecture ascompared with the scanty application in painting of the same period.Duringthe best period of Grecian art many refinements were applied in orderto correct optical illusions. It would be interesting to know to whatextent the magnitude of the optical illusions as they appeared to manypersons were actually studied. The Parthenon of Athens affords anexcellent example of the magnitude of the corrections which thedesigner thought necessary in order to satisfy the eye. The long linesof the architrave - the beam which surmounts the columns or extendsfrom column to column - would appear to sag if it were actuallystraight. This is also true of the stylobate, or substructure of acolonnade, and of pediments and other features. These lines were oftenconvex instead of being straight as the eye desires to see them.Inthe Parthenon, the stylobate has an upward curvature of more than fourinches on the sides of the edifice and of more than two and a halfinches on the east and west fronts. Vertical features were made toincline inward in order to correct the common appearance of leaningoutward at the top. In the Parthenon, the axes of the columns are notvertical, but they are inclined inward nearly three inches. They aresaid also to be inclined toward each other to such a degree that theywould meet at an altitude of one mile above the ground. The eleven-footfrieze and architrave is inclined inward about one and one-half inches.In Fig. 85, a represents the front of a temple as it should appear; b represents its appearance {exaggerated} if it were actually built like a without compensations for optical illusions; c represents it as built and showing the physical corrections {exaggerated} in order that it may appear to the eye as a does.Tallcolumns if they are actually straight are likely to appear somewhatshrunken in the middle; therefore they are sometimes made slightlyswollen in order to appear straight. This outward curvature of theprofile is termed an entasis and in the Parthenon column, which isthirty-four feet in height, amounted to about three-fourths of an inch.In some early Grecian works, it is said that this correction waso
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