INFLUENCE OF THE DIFFERENT RAYS OF THE SPECTRUM 351 



commenced. Since the concentration of the light is directly proportional to 

 the aperture, the width of the slit gives the relative assimilatory value of the 

 different rays. 



Engelmann generally employed a constant gas or incandescent electric light, 

 and calculated the results obtained in terms of the solar spectrum (1. c., 1884, p. 90) ; 

 the values obtained with prismatic spectra were corrected for the unequal dis- 

 persion in the different regions, and in this manner the curves given in Figs. 

 51 and 52 were constructed. In strong light a narrow slit may be used, giving a very 

 pure spectrum, and it may perhaps be possible to use a grating spectrum for this 

 purpose. Excellent as this method is, it yields only approximate values, and 

 owing to an improper use of it Pringsheim obtained different results to those 

 of Engelmann 1 . 



The bubble-counting method also enables comparative estimations to be made 

 of the total assimilatory effect produced by different rays (Pfeffer and Reinke), 

 especially if a microscopical method is employed by means of which the diameters 

 of the individual bubbles are measured (Kohl, Ber. d. Bot. Ges., 1887, p. in). 

 By these methods, as well as by gasometric means, the evolution of oxygen behind 

 coloured media, as well as in prismatic spectra, was determined by Draper, and 

 later by Miiller, Pfeffer, Reinke, &c. In the published works the errors due to 

 dispersion, and to the unequal absorption of different rays in the substance of the 

 prism, &c., are described. The method used by Reinke is to be recommended. 

 He blocked out regions of the spectrum determined according to Helmholtz's 

 principle, and focussed the remainder by means of a convex lens upon the plant, 

 the decreased evolution of oxygen representing the assimilatory value of the 

 absent rays. Reinke also concentrated different regions of the spectrum to strips 

 of equal size by means of cylindrical lenses, and so counteracted the unequal 

 dispersion 2 . 



Only an approximate estimation of the value of the different rays for photo- 

 synthetic assimilation can be obtained by experimenting under coloured media, so 

 long as there is no means of determining the precise extent to which the different 

 rays are absorbed. This applies even to Pfeffer's researches of 1871. The effect 

 upon the evolution of bubbles produced by interposing coloured solutions, or 

 coloured plates of glass or gelatine 3 , may be exhibited to a large audience by means 

 of a projection-lantern (Sect. 52). Solutions of cupric ammonium sulphate and 

 of potassium bichromate have been most commonly employed for this purpose. 

 After passing through both solutions sunlight retains but 8 to 20 per cent, of 



1 Cf. Engelmann, Bot. Zeitung, 1882, 1883, and 1887, pp. 100,459 ! Pringsheim, Jahrb. f. wiss. 

 Bot, 1886, Bd. XVII, p. 162 ; Bot. Zeitung, 1887, p. 200. 



2 Reinke, Bot. Zeitung, 1884, P- 2 75 Helmholtz, Ann. d. Phys. u. Chem., 1855, p. 94; 

 Reinke, Bot. Zeitung, 1885, p. 84; Ber. d. Bot. Ges., 1885, p. 378. Reinke (Sitzungsb. d. Berl. 

 Akad., 1893, p. 531) used a grating spectrum in studying the effect of different rays on the formation 

 of chlorophyll. 



3 All kinds of coloured glass plates can be obtained from Geb. Tasche in Cologne. On 

 coloured solutions, cf. Landolt, Ber. d. Chem. Ges., 1894, p. 2872. See Kirschmann, Beibl. z. 

 Ann. d. Phys. u. Chem., 1891 , Bd. XV, p. 423. Coloured gelatine plates gradually become bleached. 



