and of Indigo Light on Plants. 7 



used, and the seedlings exposed for five hours, they were so 

 inclined as to suggest the appearance of a field of growing 

 wheat, blown by two winds to a common point. If the expe- 

 riment were sufficiently prolonged, some of the plants from 

 either extremity of the spectrum interlocked in the direction 

 of the axis. 



20. This axis is in the direction taken by Fraunhofer's indigo 

 rat/ in passing from the prism to the plants. — The plants grow- 

 ing in indigo light inclined directly along it, but those of the 

 red, orange, &c. did not move towards the radiant in the 

 prism, but along a diagonal, inclined in part to the plants il- 

 luminated by the active rays, which were much nearer than 

 the prism. The amount of this lateral inclination diminished 

 as the plants were nearer the axis, so that those illuminated 

 by blue, violet and lavender were little deflected from a line 

 drawn from their place of growth to the radiant. Seedlings 

 in the red, orange and yellow rays frequently bent to such an 

 extent as to cause their summits to pass through the adjoining 

 coloured space. 



21. The secondary (lateral) inclination, remarked above, 

 did not occur when the radiant was a reflected image of the 

 spectrum, which was not allowed to fall on any of the plants. 

 If the mirror reflected neither of the more refrangible rays, 

 the plants appeared to be inclined to the light immediately 

 before them. 



22. These experiments satisfied me that the active force 

 was in the indigo ray, and the intensity of the light necessary 

 to produce deflection was extremely feeble, so that an amount 

 inappreciable to the eye (which is an admirable measure of the 

 brilliancy, but incapable of estimating the effect of quantity) 

 would, after a lengthened exposure, cause considerable deflec- 

 tion. Indeed, the phenomenon is so little dependent on the 

 brilliancy of light that very little seems to be gained by con- 

 centrating the rays beyond a certain point. There is there- 

 fore sufficient activity in each prismatic colour to produce 

 bending, if sufficient time be allowed. The movement is 

 therefore a result depending upon the absorption of light. 



23. As this is an entirely new subject, it is thought expe- 

 dient to advance some further evidence concerning the posi- 

 tion of the deflecting force. For this purpose the spectrum 

 was allowed to fall upon a screen, perforated by two similar 

 apertures, in such positions as to allow the red ray to pass 

 through one and the indigo through the other. Behind the 

 screen a box was placed containing four jars of turnep-seed- 

 lings, arranged along a line occupying the centre between the 

 intermitted rays. The light passed through the box without 

 any reflection, and was stifled by black cloth when it fell upon 



