24 PHYSIOLOGY OF NUTRITION. 



make use of the measuring apparatus, which can be illuminated 

 by means of the mirror attached to it. The comparison prism is 

 more particularly of service when it is required to compare the 

 known spectrum of one body with the not yet investigated 

 spectrum of another. If, viz., we insert the comparison prism 

 and illuminate it well, we shall perceive two spectra situated one 

 above the other, and only separated by a fine black line. When 

 the substances which we are investigating are identical, and also 

 in the same state of concentration, the bands of absorption of 

 the lower spectrum fall accurately on the prolongations of the 

 bands of the upper. 



If it is required to investigate thin sections of tissue, or even, 

 e.g., single chlorophyll grains with the micro-spectral apparatus, 

 they are placed on the slide in a drop of water or glycerine, and 

 covered as usual with a cover-glass. To focus the preparation, 

 we remove the tube carrying the prisms, and open the slit as little 

 as possible. In studying many chlorophyll-containing objects (I 

 experimented, e.g., with filaments of Cladophora), we perceive 

 only band I. in the red, and a continuous end absorption formed 

 by the fusion of bands V., VI., and VII. ; the bands II., III., and 

 IV. do not in general appear. 



In investigating entire leaves we simply lay them on the object 

 stage and use a low objective. If we illuminate our object by 

 letting diffuse daylight or gaslight fall on the mirror of the micro- 

 scope, only the bands I., II. and III. of the chlorophyll spectrum 

 are visible. With direct sunlight, bands IV. and V. also appear, 

 while bands VI. and VII., on account of the strong end absorp- 

 tion, can frequently not be distinctly made out. That the bands 

 I.-V. observed actually correspond with bands I.-V. of the chloro- 

 phyll spectrum, may be proved by employing the comparison 

 prism, which receives light from a special mirror, and with its help 

 observing simultaneously the spectrum of a leaf, and that of an 

 alcoholic solution of chlorophyll. Almost exact coincidence is then 

 found in the position of the absorption bands of the two objects ; 

 the bands of the spectrum of the solution, as compared with those 

 of the leaf spectrum, are merely displaced somewhat towards the 

 violet, a phenomenon which is caused by the solvent (alcohol) 

 used. 



To investigate chlorophyll solutions by means of pocket spectro- 

 scopes, Bunsen's apparatus, or the direct vision spectroscopes of 

 Hermann or Steinheil, we pour the solutions into glasses with 



