Design and Performance of a Biological Spectrograph

The analytical power of spectrographs and of spectrographic methods in the determination of action spectra of in vivo phenomena has only barely and recently been capitalized on by the biologist, although the theoretical importance of such determinations has long been recognized. Although there is no paucity of known photosensitive reactions, accurate quantitative data on spectral sensitivity have been collected in relatively few areas, e.g., in the mechanism of the visual response (1, 2), of microbial inactivation and change (3), and of the protochlorophyll-tochlorophyll transformation (4). One of the major obstacles has been the limited availability of physical facilities designed especially for such investigations. Numerous solutions to the instrumentation problem have been offered, of which the three following have been the most satisfactory. 1. The use of large refraction optics (prismatic dispersion) and long path lengths to ensure high dispersion, adequate intensity, and simultaneous use of the entire spectrum. The most successful application of this method has been realized in the large prism spectrograph at Beltsville (5), with which some of our best estimations of plant action spectra [e.g., for photoperiodic control of flowering (6) and for the lettuce seed-germination photoreaction (7)] have been determined. The principal limitations of such instruments are limited spectral breadth (380 to 800 m.), low energy of the blue end of the spectrum, and poor dispersion of the red. 2. The use of small prism and grating monochromators to isolate narrow spectral regions from a refracted or diffracted incident beam. Numerous accurate investigations have been performed with such instruments, which are particularly precise when the energy requirement is low and the sensitive process relatively independent of time (4). Limitations here are low intensity and short spectral availability, and the necessity of a larger number of controls when reactions are time-dependent. 3. The use of variable density filters to isolate known spectral regions from a par-