The camouflage of observation balloons has not been developed, though experiments were being considered in this direction as the war closed. Inasmuch as they are low-altitude crafts it appears that they would be best camouflaged for the earth as a background. Their enemies pounce down upon them from the sky so that low visibility from above seems to be the better choice. In the foregoing it has been aimed to give the reader the general underlying principles of camouflage and low visibility, but at best this is only a résumé. In the following references will be found more extensive discussions of various phases of the subject. REFERENCES 1. A Study of Zöllner's Figures and Other Related Figures, J. Jastrow, Amer. Jour. of Psych. 1891, 4, p. 381. 2. A Study of Geometrical Illusions, C. H. Judd, Psych. Rev. 1899, 6, p. 241. 3. Visual Illusions of Depth, H. A. Carr, Psych. Rev. 1909, 16, p. 219. 4. Irradiation of Light, F. P. Boswell, Psych. Bul. 1905, 2, p. 200. 5. Retiring and Advancing Colors, M. Luckiesh, Amer. Jour. Psych. 1918, 29, p. 182. 6. The Language of Color, 1918, M. Luckiesh. 7. Apparent Form of the Dome of the Sky, Ann. d. Physik, 1918, 55, p. 387; Sci. Abs. 1918, No. 1147. 8. Course on Optics, 1738, Robert Smith. 9. Color and Its Applications, 1915 and 1921; Light and Shade and Their Applications, 1916, M. Luckiesh. 10. Report of The Submarine Defense Association, L. T. Bates and L. A. Jones. 11. Marine Camouflage Design, E. L. Warner, Trans. I. E. S. 1919, 14, p. 215. 12. The Visibility of Airplanes, M. Luckiesh, Jour. Frank. Inst. March and April, 1919; also Aerial Photometry, Astrophys. Jour. 1919, 49, p. 108. 13. Jour. Amer. Opt. Soc., E. Karrer, 1921. The foregoing are only a few references indicated in the text. Hundreds of references are available and obviously it is impracticable to include such a list. The most fruitful sources of references are the general works on psychology. E. B. Titchener's Experimental Psychology (vol. 1) contains an excellent list. A chapter on Space in William James' Principles of Psychology (vol. II) will be found of interest to those who wish to delve deeper into visual perception. Other general references are Elements of Physiological Psychology by Ladd and Woodworth; the works of Helmholtz; a contribution by Hering in Hermann's Handb. d. Phys. Bk. III, part 1; Physiological Psychology by Wundt; E. B. Delabarre, Amer. Jour. Psych. 1898, 9, p. 573; W. Wundt, Täuschungen, p. 157 and Philos. Stud. 1898, 14, p. 1; T. Lipps, Raumaesthetik and Zeit. f. Psych. 1896, 12, 39. INDEX Aberration, 19 spherical, 122 chromatic, 135 Aerial perspective, 165, 183 After-images, 24, 25, 59, 128, 186 positive and negative, 129 Airplanes, visibility of, 233 camouflage for different types, 234 size of image at various altitudes, 238 camouflage for various conditions, 240 Alhazen, 8