6 Dr. Gardner on the Action of Light upon Vegetables. 



and seven days in beans. Plants from the field preserve their 

 color sometimes for weeks, but finally become yellow. 



(12.) The fact established by these experiments is, that the 

 less refrangible rays are most active in producing the green color 

 of plants. It is not stated, that the blue, &c. rays will not effect 

 this change in time, but, that they are remarkably inactive. 



(13.) The inaximum action is in yellow light. For the 

 purpose of obtaining a measure of the comparative activity of 

 each ray, the following experiment was made. The spectrum 

 of a circular beam of light three fourths of an inch in diameter, 

 was received upon a double convex lens of three feet focus, placed 

 near the prism. The dispersed rays passed through a chink of 

 one fourth of an inch into a camera, and each fell into a separate 

 compartment, containing a few turnip seedlings, situated near the 

 focus of the ray. The place of the extreme red and central 

 yellow rays was determined through cobalt glass, and the whole 

 spectrum divided into the spaces given by Fraunhofer, for the 

 width of each color. The arrangements being carefully ad- 

 justed, the plants were examined at intervals, by allowing a little 

 diffused light to fall upon them, and excluding the spectrum ; in 

 this way, the number of hours was obtained in which a given 

 ray produced a certain effect. The depth of green color was 

 estimated, by carefully comparing the plants, with a selected 

 specimen ; in this I was assisted by a friend, whose eye is well 

 skilled in distinguishing between shades of color. 



(14.) The best result gave for the yellow 3£ hours, the orange 

 4£ hours, and the green ray 6 hours; the plants were selected 

 from the centre of each group, and all the measures obtained on 

 the same day during uninterrupted sunshine. The experiment 

 was continued until 17£ hours of sunlight had acted upon the 

 seedlings in the blue space, which then acquired a tint estimated 

 at one half that of the test. In another observation the indigo, 

 violet, and lavender of Sir John Herschel produced no effect in 

 23 hours. 



(15.) From these experiments I conclude that the centre of 

 the yellow ray is the point of maximum effect in the production of 

 chlorophyl ; and that the action diminishes on either side to the 

 termination of the mean red and blue. 



(16.) In this stage of the subject, an interesting question sug- 

 gests itself — is the active agent light ? some form of chemical 

 ray ? or heat ? 



