154 



JS/ATURB 



[Jtmee, 1878 



It is a curious coincidence that while I was at 

 work on Drosera, an almost identical research was 

 being conducted in Germany. The experiment of Drs. 

 Kellermann and von Raumer were described before the 

 Phys. Med. Society of Erlangen in July, 1877, and the 

 final results were communicated by Rees, of Erlangen, to 

 the Botanische Zeitung, April 5, 1878. 



The research was evidently conducted Avith extreme 

 care, and it is very satisfactory to me to find that 

 my results agree (speaking generally) with those of 

 Kellermann and von Raumer. The plants used in their 

 experiments were fed on aphides, and do not seem to 

 have thriven quite so heartily as mine did on a meat diet. 

 This appears from the following figures : — 



Number of flower stems 

 Number of capsules 

 Weigrht of seeds 



Kellermann and 



von Raumer's 



results. 



100 : 152 

 100 : 174 

 100 : 20$ 



Mine. 



100 : 165 

 100 : 194 

 103 : 380 



In testing the relative powers of the fed and not fed 

 plants in laying by reserve-material in the winter buds, 

 the Erlangen observers adopted a more accurate method 

 than mine, namely, that of weighing the winter buds, 

 instead of waiting until the new leaves had grown. They 

 found that the weights of winter buds for the fed and not 

 fed plants were as 173 : 100. FRANCIS Darwin 



PHYSICAL SCIENCE FOR ARTISTS ^ 

 V. 



THE simple and forcible language employed by Prof. 

 Stokes in the extract I gave in my last paper, should 

 have made it quite clear that in nine cases out of ten, 

 when bodies reflect light to us, they have really absorbed 

 a part of it in the process, and that to this absorption of 

 light bodies by their colours are chiefly to be ascribed. 



Those bodies which give back to us light in the middle 

 of the spectrum — light, in other words, containing green 

 and yellow — are those which are most liable to change 

 with different intensities of light. I shall endeavour, if 

 I have space, to return to this point in the sequel, but I 

 feel that my first duty, now that the phenomena of ab- 

 sorption have, I trust, been clearly explained, is to pass 

 on to the application of this knowledge to the various 

 colours of the sky. 



Having, then, this idea of absorption, a very important 

 consideration comes in : the absorption of a substance 

 generally increases with its thickness, and when we deal 

 with those substances that for a given thickness absorb 

 either the red or the blue, we often find that when the 

 thickness is considerably increased the absorption spreads 

 over the whole spectrum from the blue or the red end 

 respectively. This can be shown graphically as follows : — 



Blue Absorptioft. 



a [§ © V ® ^i'^'^or^" 



V 



Q © © V ® 



V I © © V © [^ 



V I B © "^ © [^ 

 VI BGV © [g 

 VI B G Y © [JS 



V I B G Y O [^ 







Here, then, we have an absorption beginning at the blue 

 end and gradually closing everything except the red. I 

 may remark en passaftt that we here have the physics of 

 sunrise and sunset colours ; similarly we might begin at 

 the red end and then we should get , 



W D 



^ a 

 w a 

 w a 



© V © R 



©¥ 

 ©Y 



OR 

 OR 



GYOR, 



and so on. 



;w These effects may be experimentally observed by either 

 using different thicknesses of the absorbing materials or 

 by putting them into a V-shaped vessel, and observing 

 the change which takes place when the light passes 

 through the greatest and least thicknesses of the absorb- 

 ing material. It is of importance also for the artist to 

 observe the effect of the residual light independently of 

 the spectral phenomena. For instance : if we take a 

 chlorine tube of such a length that it begins to cut off the 



Lea«t 

 thickness 



Pvt.usium permanganate. 



Chromx chloride. 



FiG.'i. — Showing phenomena of absorption produced by great and small 

 thicknesses of the same substance in a wedge-shaped cell. 



blue the residual light will be a delicate green ; a tube 

 twice the length will give us a colour in which the rich 

 golden yellow predominates. 



Although we have been compelled to leave out several 

 steps in the argument, we are in a position now to ap- 

 proach the cause of the various colours of the sky. 



Let us assume that our complex atmosphere— complex 

 because it consists of a mixture of two pure gases and 

 aqueous vapour— absorbs the light which passes through 

 it, and'that the absorptive effect depends upon the thick- 

 ness o"? the atmosphere through which the light has to 

 pass before it reaches the eye. 



Now there are many grounds for supposing that the 



Ccnt.nued from p. 126. 



Fig. 2.— Arrangement for observing the absorption of a great thickness of 

 gas or liqu.d. L, lamp; N, tube ; s, spectroscope; g, bunsen burner 

 when used. 



general absorptive effect both of the pure gases and of 



