Very long optical paths in air

The paper describes two different multiple traversal optical systems for increasing the number of passes a large aperture beam can make through a restricted volume by the formation of a rectangular array of image spots on the single mirror of a White cell. The system for the longest path lengths uses seven mirrors in a near focal arrangement. A rectangular array of images corresponding to different numbers of passes appears on four mirrors at one end of the system, while, at the other end, three mirrors form the array and illuminate each image in it from one or more different directions. In the second system, the system for maximum stability with the best compensation for vibration and misalignment problems, the addition of four diagonal mirrors to a White cell converts the two lines of images on the single mirror to a rectangular array of images, almost squaring the maximum possible number of passes.