NO. 12 SPECTROBIOLOGY BRACKETT AND McALISTER 7 



stituted for this work. Because of its large area and uniform response 

 this arrangement proves quite satisfactory. It is, of course, particularly 

 suitable to the algal plate. As the same set-up of the photometer was 

 used with the photographic plate, considerable of the detail is not 

 brought out. 



This combination of equipment, spectrograph and microphotometer, 

 makes possible a type of biological investigation which is capable of 

 yielding quantitative results, approaching those of photographic spec- 

 trophotometry, the limit being set simply by the uniformity of tex- 

 ture of the biological plates which can be prepared. 



N-,A, 



3000 



Fig. 5. — Photometric record of photographic spectrogram of mercury arc. 



Obviously the equipment developed presents unusual advantages 

 for pure photochemical investigation. Because of the large aperture 

 and long slit it is possible to secure a large amount of monochromatic 

 energy, thus enabling one to work with large enough quantities of 

 material for convenient analysis. A mounting has been developed 

 which permits the simultaneous exposure of 6 small test tubes to 

 different lines of the spectrum. As in many cases slit-widths as great 

 at 2 mm are possible without overlapping of lines, amounts of energy 

 become available of the order commonly secured in photochemical 

 practice by technique which yields considerably less resolution. Thus, 

 with lines varying in intensity from 4.8/ergs/sec./mm ' to 125.6 ergs/ 

 sec/mm', and slit 2 mm wide by 50 mm long, one obtains corres- 

 ponding rates of energy supply 480 ergs per sec. to 12,560 ergs per sec. 

 Each test tube is surrounded by its own cylindrical cover in which a 

 slit of the desired width has been cut. This second use of the instru- 

 ment has already proved of considerable interest, cooperative work 

 having been carried out with the United States Department of Agri- 

 culture upon photochemical changes in plant products. 



