Dr Gardner on the Aetion of Light on Vegetables. 91 



thence was feeble, with, two minima at — 5.0 and + 6.8, with 

 a slight intermediate maximum at (0.0) the centre of the yel- 

 low, and beyond these, or about the termination of the green, 

 the action again increases, reaches another maximum at + 20.0, 

 which corresponds to the centre of Fraunhofer's indigo, after 

 which it declines to a point beyond the violet + 45.0. 



I have been thus precise in giving his result, because my 

 experiments made with ethereal solution of chlorophyl from 

 grass leaves spread upon paper, gave similar spectra. There 

 are two points, however, which it is necessary to discuss. 



The first action of light is perceived in the mean red ray, 

 and it attains a maximum incomparably greater at that point 

 than elsewhere ; the next place affected is in the indigo, and, 

 accompanying it, there is an action from + 10.5 to + 36.0 

 of the same scale, beginning abruptly in Fraunhofer's blue. 

 So striking is this whole result, that some of the earlier spec- 

 tra obtained by me, contained a perfectly neutral space from 

 — 5.0 to + 10.5, in which the chlorophyl was in noway 

 changed, whilst the solar picture in the red was sharp and of 

 a dazzling whiteness, and the maximum of the indigo was also 

 bleached, producing a linear spectrum, as in fig. 2 ; in which 

 the orange, yellow, and green rays are inactive ; these, it will 

 be remembered, are energetic in forming the green matter. 



Fig. 2. 



Red, orange, yellow, green, blue, indigo, violet. 



Upon longer exposure, the subordinate action along the yel- 

 low, &c., occurs, but not until the other portions are perfectly 

 bleached. 



In Sir J. Herschel's experiments, there remained a salmon- 

 colour after the discharge of the green. This is not seen when 



of the yellow, and the termination of the violet is sharp and distinct. Sir 

 John Herschel takes the centre of the yellow, thus insulated, as his zero 

 point, and, using a scaleof thirtieths of an inch, divides the distance between 

 it and the red end into negative parts, and in the direction of the violet po- 

 sitively. The spectrum he used contained 13.30 negative, and 40.62 po- 

 sitive parts, and was, therefore, ^^i inches long. My spectrum corre- 

 sponded with this very closely. 



