THE FOOD OF PLANTS. 



25 



parallel walls, such as are to be obtained, e.g., from Muencke, 

 Berlin (see Fig. 7) ; or what is best, transfer them to a hfemoscope 

 (to be obtained of Desaga, in Heidelberg), so as to be able to 

 examine in rapid succession layers of fluid of different thicknesses. 

 If a gas or petroleum flame is used as the source of light, it fre- 

 quently happens that we can only make out distinctly bands I. and 

 II., or these together with band III.* Accurate spectroscopic in- 

 vestigation of chlorophyll can be made in direct sunlight. The 

 spectroscope is placed in a dark room. The light enters through 

 an opening in the window shutters, in front of which, in order to 

 keep the direction of the rays of light constant, is placed a helio- 

 stat, whose mirror may be moved by hand, or better, rotated by 

 means of clockwork. Good heliostats are supplied by Ehrhardt 

 & Metzger, Darmstadt. In the dark room 

 the walls, floor, ceiling, window shutters, 

 tables, etc., are to be painted dull black. 

 The absorption bands in the more strongly 

 refrangible part of the chlorophyll spec- 

 trum are only to be seen clearly separated 

 from one another when we experiment 

 with comparatively dilute solutions. The 

 results are to be noted down in accordance 

 with the observations made with the help 

 of the scale of the spectroscope. 1 



It has been indicated in 6 that normal 

 green chlorophyll is a mixture of two 

 colouring matters xanthophyll and cy- 

 anophyll. The method to be employed 

 for separating these two colouring matters 

 from one another has also already been 

 described. The solution of the yellow 

 xanthophyll (at least the dilute solution) 

 exhibits on spectroscopic examination 

 only three absorption bands, all situated in the blue and violet. 



* Very bright spectra are also given by the pocket spectroscopes of Browning, 

 which are to be obtained, provided with scale, from Schmidt & Hansch, in 

 Berlin. As a source of light we may use an Argand burner with a tin chimney. 

 These spectroscopes are now frequently employed for studying the absorption 

 spectrum of chlorophyll solutions. It is possible, viz. with these, to determine 

 quite accurately the position of the bands I.-IV. See Wegscheider, Ber. d. 

 Dcntsch botan. Gesellschaft, Bd. 2 ; and Vogel, Praktische Spectralanalyse, Nord- 

 lingen, 1877. 



FIG. 7. Glass vessel with 

 parallel walls for the recep- 

 tion of fluids to be examined 

 spectroscopically. 



