82 SUMMAEY OF CURKENT RESEARCHES RELATING TO 



1. Directly, by modification of the interchange of gases, without 

 apparent addition of a sensation. 



2. By modification of the sensation of necessity for breathing, 

 owing to modification of the interchange of gases. 



3. By setting up a si)ecific process, which probably answers to our 

 sensation of light. 



Of these, the first may occur either alone or in combination with 

 the second ; simultaneous occurrence of the first and third may pos- 

 sibly take j)lace. 



1. Navicula is taken as type of the first method, and most mobile 

 Diatomacea3 and Oscillarine8B belong to the same class, Pinnularia 

 was also examined. Movement is here intimately connected with the 

 presence of free oxygen, which, if not present, can be produced by 

 these organisms in the light. It is for this reason that light is able 

 to revive the movements when they have ceased through want of 

 oxygen in the darkness, whereas when sufficient oxygen is present 

 already in the water, the light exercises no distinct influence on the 

 energy of the movements. After movements have ceased owing'to want 

 of light, they may be made to recommence by placing the diatom in 

 the red part of the spectrum. It is found that red between the lines 

 B and C promotes movements the most actively, while ultra-red and 

 ultra-violet are quite ineffective. The relation to the colours of the 

 spectrum, together with the amount of light required (which is con- 

 stant), agree with those indicated by the bacterium test. Taking that 

 of the red between B and as the maximum, the percentage of energy 

 developed by the other colours is : — 



Extreme red (a) .. .. 22-7 



Green (E^h) .. .. 14-1 



Extreme blue (F) .. .. 6-9 



Violet (G) 1-2 



2. ParamceciUm bursaria. — When the proportion of oxygen is 

 normal, or somewhat greater than the normal amount, the Infusorian 

 is usually very quiet ; if, however, it sinks ever so little below this 

 degree, the animal becomes restless, and makes for places in which 

 there is more oxygen (e. g. edge of cover-glass) ; in good light, but 

 under otherwise similar conditions, the specimens distribute them- 

 selves equally throughout the drop. Active swimming is the con- 

 sequence of serious diminution of the oxygen ; if strong light is then 

 applied for some minutes, the Paramcecium courses rapidly about, and 

 if insufficient supplies of oxygen are added from without, it shows 

 itself very sensitive to alterations in the illumination in the spectrum ; 

 it prefers red of between the lines B and C, The obvious explanation 

 of this and other details is that, in default of oxygen from without, the 

 chlorophyll contained in the mesoplasm acts as it does in plants — 

 viz. excretes oxygen ; it exhibits in its action the same dependence 

 on amount and quality of light as do the movements of the animals. 

 The energy of the excretion is as follows for different parts of tho 

 spectrum, taking the red between B and C as 100 : — 



