While studying the infrared spectra of molecules adsorbed on solid surfaces, it was desirable to have the sample placed at a focal point in the infrared beam and not at the normal sample position used in the Model 21. Moreover the sample space in the instrument is 12 cm and it was necessary in this work to accommodate samples with path lengths up to 25 cm, when equipment was used for low or high temperature infrared studies. The simple optical system, shown in Figure 1 (a), was constructed to meet these requirements. In Figure 2, the optical system, with a sample situated at the focus, is shown attached to the spectrometer.
L. H. Little, "Supplementary Optics For Perkin-Elmer Model 21, Double Beam Infrared Spectrophotometer," Appl. Spectrosc. 15, 83-83 (1961)