Figure 2. Diagram of Model DU Optical System from Beckman Instruments Instruction Manual / The Beckman Model DU Spectrophotometer and Accessories Beckman Division, Beckman Instruments, Inc. Fullerton, CA, [1954]. Back cover illustration states March, 1954 No copyright statement. No copyright renewals.

Full caption to Figure 2, page 3, reads as follows:

“Light from the tungsten lamp is focused by the condensing mirror and directed in a beam to the diagonal slit entrance mirror. The entrance mirror deflects the light through the entrance slit and into the monochromator to the collimating mirror. Light falling on the collimating mirror is rendered parallel and reflected to the quartz prism where it undergoes refraction. The back surface of the prism is aluminized so that light refracted at the first surface is reflected back through the prism, undergoing further refraction as it emerges from the prism. The desired wavelength of light is selected by rotating the Wavelength Selector which adjusts the position of the prism. The spectrum is directed back to the collimating mirror which centers the chosen wavelength on the exit slit and sample. Light passing through the sample strikes the phototube, causing a current gain. The current gain is amplified and registered on the null meter.