A CHEMICAL PHOTOMETER 239 



the intensity of the rays at one end of the spectrum to a greater 

 extent than those at the other end. It may be stated, however, 

 that it was thought advisable by Professor Kimball to discard 

 some of the comparisons made because of the presence during 

 the exposure of moving cumulus clouds which caused the record- 

 ing pen of the pyrheliometer to move so rapidly that accurate 

 evaluation of its curve was impossible. Another explanation 

 of apparently discordant results lies in the fact that instruments 

 of the recording pen type are liable to considerable error by lag- 

 ging, especially when actuated by widely different impulses of 

 short duration occurring in rapid succession, such as the effect 

 of moving clouds just mentioned. 



Until the question of the specificity of physiological effect 

 upon plants of light rays of different wave lengths is settled to 

 the satisfaction of plant physiologists, it seems to the writer 

 that a means of light measurement in general by a chemical 

 method is greatly to be desired. The spectrum of a chlorophyll 

 solution shows definite absorption bands in the red and orange 

 and almost general absorbtion in the blue, indigo and violet 

 with smaller bands interspersed in other portions. The pro- 

 jection of the solar spectrum for some hours upon a leaf has 

 demonstrated that photosynthesis takes place most prominently 

 in the region of the red and to some extent at other points, 

 though few acceptable results indicating the effect of light of 

 various wave lengths upon other life processes of the plant 

 have been stated.^ As far as light is concerned, physiological 

 investigations deal, in the main, with that factor in its totality, 

 its effect upon plants being generally regarded as photochemical; 

 hence, the feasibility of the chemical method of measurement 

 herein described, should future investigations confirm its seem- 

 ing usefulness. 



5 Richter, A. (in Rev. Gen. de Bot., 1902, p. 212) indicates that the amount 

 of photosynthesis in a leaf subjected to monochromatic light is a function of the 

 heat energy of that light and independent of its wave length.. Recent articles 

 published in the Journal of Experimental Zoology by Dr. S. O. Mast deal with 

 the stimulating effect of different spectral colors upon lower organisms. The 

 bactericidal action of ultra-violet rays, which must be a function of wave length 

 rather than heat, is well established, Ayers, et al., loc. cit. 



THE PLANT WORLD, VOL. 21, NO. 9 



