and of Indigo Light on Plants. 5 



vender portions were always inactive, although several experi- 

 ments were continued until the plants faded. 



11. Under favourable circumstances it requires a long ex- 

 posure to develope chlorophyl. The shortest period I wit- 

 nessed was in a crop of turnep-seedlings, which required two 

 hours in the centre of the yellow, but frequently six or more 

 hours were necessary. In a full Virginia sunshine it requires 

 more than an hour to produce the same effect. 



The colour acquired is not fugitive. It has been observed 

 scarcely changed after seventy-two hours' darkness in turneps, 

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

 colour sometimes for three weeks, but finally become yellow. 



1 2. The fact established by these observations is, that the 

 less refrangible rays are most active in producing the green 

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

 not effect this change in time, but that they are singularly in- 

 active. 



13. The maximum action is in the yellow light. — For the 

 purpose of obtaining a measure of the comparative activity of 

 the different rays, 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 a quarter of an inch into a camera, and 

 each fell into a separate compartment, containing a few turnep- 

 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 colour. The 

 arrangements being carefully adjusted the plants were exa- 

 mined 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 amount of colour. The depth of green was esti- 

 mated by carefully comparing the plants with a selected spe- 

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

 skilled in distinguishing between shades of colour. 



mi CO 



1 4. The best result gave for the yellow 3^ hours, the orange 

 4| hours, and the green 6 hours ; the plants were selected from 

 the centre of the groups, and all the measures obtained on the 

 same day during uninterrupted sunshine. The observation 

 was continued until 17y hours sun-light had acted upon the 

 plants in the blue, which then acquired a tint estimated at one- 

 half that of the test. In another experiment, the indigo, violet 

 and lavender spaces exhibited no plants which had changed 

 in 23 hours. 



