150 



BIOLOGICAL EFFECTS OF RADIATION 



used for the particular region of the spectrum. (Grating instruments 

 are seldom used for photochemical or biological purposes, where high 

 intensity is required.) For example, a single quartz monochromator 

 yielded a transmission of 40 per cent at 2250 A and rose to 50 per cent 

 at 6000 A. 



Fig. 6. — Monochromatic illuminators, (a) Single monochromator, showing con- 

 densing system. (6) Double monochromator. Two single monochromators may be 

 combined in tandem. Where a special instrument is built, field lenses FFi are used, wave- 

 length change being accomplished by a motion of slit Si. 



In addition to the radiation which forms a part of the true image 

 falling on the second slit, radiation of other wave-lengths also falls upon 

 the slit, which arises from surface imperfections, bubbles, and inhomo- 

 geneities of the refractive material within the optical parts, together with 

 light returned to the beam by surfaces not intended to contribute to the 



