GENERATION, CONTROL, AND MEASUREMENT 211 



are nearly always used. The echelette grating would be suitable for a 

 double monochromator only near the spectral region of the blaze. 



Spectroscopic Optical Arrangements. Alost spectroscopic optical ar- 

 rangements or mountings employ a collimator to provide parallel flux 

 incident to the prism or grating. The notable exceptions are the Fery 

 prism, the concave grating mountings, and the converging flux mountings 

 for the prism and plane grating. Several typical optical arrangements 

 for mountings for prism and grating monochromators are diagrammati- 

 cally presented in Fig. 3-24. 



The principle of a single lens or mirror with converging rays on the 

 prism or plane grating has been little used in large spectroscopes and 

 irradiation monochromators but deserves much more attention than it 

 has received. One such arrangement for the prism is described by Parker 

 et al. (1946) and has been employed for obtaining action spectra of the 

 photoperiodic responses in plants. This type of mounting for the plane 

 grating was originally described by Monk (1928) and later redescribed in 

 greater detail by Gillieson (1949). It has the advantage that the slit 

 image can be highly magnified into a large spectrum with only one optical 

 element. There is probably less stray flux than with the autocollimating 

 arrangements, and almost any degree of magnification of the spectral 

 image can be obtained. 



Methods of Irradiation. There are two general spectroscopic methods 

 of monochromatic irradiation: (1) the spectrograph method, in which a 

 spectrograph is used to irradiate a series of objects at one time, each with 

 a different dominant wave length selected from the spectrum; and (2) 

 the monochromator method, in which a monochromator is used for the 

 irradiation of a single object at one time with homogeneous monochro- 

 matic energy of a single dominant wave length. When the biological 

 material is in the form of small particles, as in the case of viruses, bac- 

 teria, or spores, the spectrograph method may be used by spreading the 

 material on a glass plate inserted in the camera. For large objects, 

 such as small animals or plants, a highly magnified spectrum is required 

 (Parker et al., 1946). 



The spectrograph method has the advantages that the spectrum can be 

 covered rapidly and that many individual treatments of diverse wave 

 lengths can be made simultaneously on samples from the same popu- 

 lation. The method is especially useful where long irradiation periods 

 are required. However, the use of a series of segments of the full spec- 

 trum is limited to the single spectrograph, and no double monochromator 

 or predispersion system can be employed before the entrance sUt. Thus 

 the background radiation due to scattering may be significant and not 

 readily eliminated. 



With the monochromator method it is much easier to isolate a narrow 

 region of known spectral composition, and predispersion systems can be 



