Dr Gardner on the Action of Light on Vegetables. 81 



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

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

 colour 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 

 colour 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 maximum action is in yellow light. For the pur- 

 pose 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 colour. The arrange- 

 ments being carefully adjusted, the plants were examined at 

 intervals, by allowing a little difi'used 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 colour was estimated by carefully compar- 

 ing the plants with a selected specimen ; in this way I was 

 assisted by a friend, whose eye is well skilled in distinguishing 

 between shade, of colour. 



(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 gi'oup, 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 those experiments, I conclude that the centre 

 of the yellow rag is the point of maximum effect in the pro-" 



VOL. XXXVII. NO. LXXIII. JULY 1844. » 



