2 Dr. Gardner on the Action of Yellow 



active. M. Morren, in 1832, and Dr. Daubeny (Phil. Trans. 

 1836) arrived at the conclusion that the activity was as the 

 luminous power of the rays respectively. The next investi- 

 gator, Dr. Draper (Journal of the Franklin Institute, 1837), 

 obtained better results in yellow than blue light. Mr. Hunt, 

 in 1840 (Phil. Mag., Apr.), resumed the question, and pub- 

 lished the most decided results (p. 272), to the effect, that blue 

 light alone causes the green colour of plants, and that the 

 yellow and red rays "destroy the vital principle in the seed." 

 In 1841 he was one of a committee appointed by the British 

 Association to report on this subject, and in a subsequent con- 

 versation, at the late meeting of that body, has repeated his 

 statements. Being the last writer, his results have given a 

 prominence to the doctrine that chlorophyl is produced by 

 the blue rays, so as to mislead Prof. Johnston in his Agricul- 

 tural Lectures, and Prof. Graham (Chemistry, p. 1013). 



4. In September 1840 I repeated Mr. Hunt's experiments 

 in Virginia, and obtained dissimilar results. A known number 

 of turnep-seeds were sown, and every grain germinated in the 

 yellow and red rays. The greenest plants were found in 

 yellow light. Every condition was favourable and the results 

 pronounced, but my reason for deferring the publication arose 

 from a conviction that the use of solutions and coloured glasses 

 was objectionable, and that no perfect results could be obtained 

 except with the spectrum. Plants exposed to light which has 

 permeated cobalt glass, are not placed in the blue rays, but 

 in red, yellow, green, indigo and violet, in proportions differ- 

 ing with the tone of colour and thickness of the material. The 

 effect may therefore be produced by any of these rays, or by 

 their peculiar combination*. 



5. I shall not attempt to explain the discrepancy between 

 my results and those of Mr. Hunt, for I do not esteem re- 

 searches made with coloured media of any value in this branch 

 of vegetable physiology. It is well to remark, however, that 

 in treating of the germination of cress-seed behind the blue, 

 green, yellow and red media, he states, " that the earth con- 

 tinued damp under \\\e green and blue fluids, whereas it rapidly 

 dried under the yellow and red" (p. 271). This difference 

 would have been considered sufficient to retard or " destroy ?" 

 germination by most persons. 



6. Other engagements in 1842 interfered with my design 

 of examining the question with the spectrum, and it was not 

 until July 1843 that such arrangements were made as are ne- 

 cessary to the prosecution of the subject. 



7. The Apparatus.— A beam of the sun's light was directed 



* See Sir J. F. W. Herschel, Phil. Trans., part i. 1840, on the «' Combined 

 Action of Rays of different degrees of Refrangibility," p. 24. 



