Introduction I. An Elucidation of the Fourth Dimension II. Non-Euclidean Geometry and the Fourth Dimension III. Fourth Dimension Absurdities IV. The Boundary of the Four-Dimensional Unit and Other Features of Four-Dimensional Space V. How the Fourth Dimension May Be Studied VI. Space and Hyperspace VII. An Interpretation of the Fourth Dimension VIII. Length, Breadth, Thickness, and Then What? IX. The Fourth Dimension Algebraically Considered X. Difficulties in Imagining the Fourth Dimension XI. Some Fourth Dimension Curiosities XII. Characteristics of the Fourth Dimension XIII. The Fourth Dimension, the Playground of Mathematics XIV. The True and the False in the Theory of Four Dimensions XV. The Ascending Series of Dimensions XVI. The Mind's Eye and the Fourth Dimension XVII. Other Dimensions Than Ours XVIII. The Meaning of the Term "Fourth Dimension" XIX. A Pupil in Geometry Quizzes His Teacher About the Fourth Dimension XX. Possible Measurements and Forms in a System of Four Dimensions XXI. The Fairyland of the Fourth Dimension XXII. The Properties of Four-Dimensional Space