With spectrographs of high dispersion it is impractical to record the whole spectrum from 2,000—10,000 A on one continuous photographic plate or film. For this reason many large spectrographs are made readily adjustable to focus any selected wavelength range on to a standard size photographic plate, generally 10" × 4". The situation can arise where plates exposed on different but overlapping wavelength ranges have to be compared, but the lateral adjustment on standard comparators is insufficient. In this laboratory an early model Judd-Lewis Comparator has been permanently adapted and used for a number of years for this purpose. Figure 1 shows the attachment in position on a current model.
H. P. Rothbaum and H. J. Todd, "Modifications to Spectrum Plate Comparator to View Overlapping Wavelength Ranges," Appl. Spectrosc. 15, 82-83 (1961)