ACTION OF LIGHT OF VARIOUS COLOURS. 303 



force itself upon the investigator whether, and in what manner these different rays 

 of light, of which daylight is made" up, influence assimilation in the chlorophyll. 

 For the preliminary guidance of those not quite familiar with the physical knowledge 

 appertaining here, the following remarks may be made. If the sun's rays are 

 allowed to fall through a narrow slit in the shutter of a room, they pro- 

 ceed through space in the form of a straight band, which can easily be seen as 

 luminous striae in the dusty air: if these luminous striae or bundles of rays are 

 allowed to pass through a triangular glass prism, the edges of which we suppose 

 vertical, two results follow. First, the ray of light is diverted from its straight path—* 

 it falls on quite another spot on the hind wall of the chamber than was the case 

 in the absence of the prism ; and secondly, instead of the one bright stripe which 

 the solar rays originally formed on the hind wall, there now appears a horizontal 

 coloured band, the so-called solar spectrum, in which the colours of the rainbow, red, 

 orange, yellow, green, blue, and violet, follow one another in such a way that 

 the red portion is least, and the violet most strongly diverted from the rectilinear 

 path of the beam of light. In this spectrum, by proper management, a number 

 of black lines appear, running perpendicularly in the horizontal band of colours; 

 these are the so-called Fraunhofer's lines, which, as KirchofF and Bunsen have 

 shown, are produced by the absorption of certain rays of hght by the incandescent 

 vapours of certain metals in the solar atmosphere. From these fixed lines in the 

 solar spectrum, the most evident of which are distinguished by the letters A, B, C, — 

 ZT, it is possible to determine exactly the place where definite effects occur. The 

 refrangibility and colour of the different parts of the spectrum are a consequence, as 

 the science of optics teaches, of the different wave-lengths in the vibrations of the 

 luminous aether, of which the light consists. 



If now the solar rays, passing through the slit, are allowed to traverse a glass 

 vessel with parallel walls containing a dark blue solution of ammoniacal oxide 

 of copper, the whole of the red and yellow, and part of the green bands in the 

 spectrum disappear; the blue solution has absorbed, kept back, or destroyed 

 these constituents of the sun-light. If a vessel with a concentrated solution of 

 bi-chromate of potash, which appears to our eyes of a deep orange colour, is 

 placed at the same spot, just those parts of the spectrum are cancelled which 

 previously passed through the blue solution — i. e. we now see in the spectrum the 

 red-orange, yellow, and a part of the green, while the blue and violet have 

 disappeared. We have thus in these two fluids excellent means for cutting out 

 the one or the other half of the solar light; and we can therefore, with the aid 

 of these two solutions, experimentally answer the question, what effect does the 

 red-yellow or the blue-violet half of the spectrum respectively exert in the de- 

 composition of carbon dioxide ? After the preliminary and less instructive researches 

 of Daubeny (1836), I made in 1864 a detailed investigation with regard to this 

 question^. In a glass cylinder filled with water containing carbon dioxide a 

 water-plant was placed ; at the cut surface of the stem of this the oxygen evolved 

 under the influence of light escaped regularly in the form of bubbles. This cylinder 



' Sachs, ' Wirkungen farbigen Lichtes auf Pflanzen' Bot. Zeit. 1864, p. 353, where the older 

 literature also is collected. 



