18 May 1984
Vol 224, Issue 4650

About The Cover

Color disks and machine to rotate them. James Clerk Maxwell's 1849 theory of color was studied with the use of such disks. Appropriate combinations of red, blue, and green gave the various colors as the wheels were spun. The apparatus shown here, from the University of Pavia, was made around 1865, probably in Italy. [From G. L'E. Turner, Nineteenth-Century Scientific Instruments, reviewed on page 731]