Irradiation Illusion
The Helmholtz 
in the 19th century. Despite the fact that the twoabove figures are identical in size, the white hole looks bigger than the black one in this Illusion.
References
Pappas, T. ``Irradiation Optical Illusion.'' The Joy of Mathematics. San Carlos, CA: Wide World Publ./Tetra, p. 199, 1989.
- Coxeter Group
- Coxeter Matrix
- Coxeter's Loxodromic Sequence of Tangent Circles
- Coxeter-Todd Lattice
- Cox's Theorem
- Cramer's Rule
- Cramér Conjecture
- Cramér-Euler Paradox
- Cramér's Theorem
- Craps
- CRC
- Creative Telescoping
- Cremona Transformation
- Cribbage
- Criss-Cross Method
- Criterion
- Critical Line
- Critical Point
- Critical Strip
- Crook
- Crookedness
- Cross
- Cross-Cap
- Cross-Correlation
- Cross-Correlation Coefficient