3o6 LECTURE XVIII. 



the accompan}-ing figure, in which are represented the effects of the different parts 

 of the spectrum, not only on assimilation in the chlorophyll, but also on the heating 

 of a thermometer as well as on the decomposition of silver salts, and, finally, 

 on the eye. It is seen at once that the evolution of oxygen, or Draper's curve, 

 cannot possibly be due to heating, or to the ordinary chemical effect of the 

 spectrum ; since the former reaches its maximum beyond the red end, and the 

 latter in the violet. However, the remarkable fact that the curve of brightness in 

 the spectrum nearly coincides with Draper's curve, has led some to the erroneous 

 assumption that Draper's curve itself is an effect of brightness ; such an idea is 

 simply without meaning, since by the word brightness we mean in this case nothing 

 more than the action of light on the human eye, an effect which may possibly 

 be quite different on the eyes of different animals. It is obvious that the action 

 of light upon our eye cannot possibly be the cause of the action of light on 

 cells containing chlorophyll. The entire assumption depends therefore upon 

 want of thought. I expressed the matter of Draper's curve correctly in the third 

 edition of my ' Text-book ' in the following words : the evolution of oxygen 

 brought about by the chloroph\ll is a function of the wave-length of light, so 

 that only light of wave-lengths which are not larger than xö^^-ö-ö rn"^- ^■i^d not 

 smaller than j-jj^^öö mm. bring about the evolution of oxygen. Proceeding from 

 both extremes, the effect of light on the evolution of oxygen ascends when its 

 wave-lengths approach xö^^öö "^™- where the maximum effect lies. Or, if we 

 start with the medium wave-lengths of the coloured regions of the spectrum, 

 measured in hundred-thousandths of millimetres, the evolution of oxygen is 

 effected by light-waves the minimum length of which begins at about 40; it 

 increases as this ascends to about 59, and decreases again as the wave-length 

 increases, becoming almost nil at a wave-length of about 69. We have thus a 

 phenomenon resembling the case of the curve of temperature, first established by 

 me : as in that case the functions of the plant which depend upon temperature only 

 begin with a certain intensity of the heat-vibrations, and ascend as the intensity 

 increases, until at an optimum temperature the maximum effect occurs, falling 

 again to zero with a further increase of temperature ; so also in the case of 

 Draper's curve, we see that the evolution of oxygen in plants begins at a certain 

 small wave-length, and that a wave-length of tüotö^ ™''^- represents about the 

 optimum, wehere the maximum effect occurs, and that with a further increase 

 of the wave-length the physiological effect falls to zero. 



Although these results have apparently only a purely theoretical interest, it 

 is nevertheless not to be forgotten that they may be of practical value in the 

 criticism of certain vital phenomena of plants. Plants for example which grow 

 in the deep shade of woods, meet with light which has to a great extent passed 

 through the translucent leaves of trees, and which has lost a great part of its 

 yellow, blue, violet, and ultra-violet rays in the chlorophyll of the latter ; and in 

 like manner plants growing in the depths of large seas receive light of totally 

 different composition from that received by terrestrial plants, and to which the highly 

 refrangible rays especially are wanting. Again, it is futile to attempt to improve 

 greenhouses with blue glass, proceeding from the erroneous assumption that the 

 plants will thus have chiefly blue light conveyed to them, and from the further 



