IRRITABILITY : INFLUENCE OF LIGHT 199 



arranged themselves in greatest concentration in the infra-red 

 (between hnes * 80 and 90). There was a distinct but smaller 

 concentration in the orange-red (between lines 54 — 64), and a 

 still smaller gathering in the green (between lines 52 — 55), whilst 

 a few also collected in the hlue-violet. The organisms avoided 

 the other colours. It is shown below (p. 200) that the region 

 of maximum concentration of these bacteria, namely the infra- 

 red, is also the region of maximum absorption of energy. Engel- 

 mann considered that the correspondence was significant, 

 and that oxygen was liberated in that zone because photo- 

 synthesis was carried on by the purple bacteria. Here, 

 however, different wave-lengths were utilized for the absorption 

 of light energy. The significance of these results will be dis- 

 cussed later. 



Effect of Colour on the Absorption of Light. 



In order to examine the absorption spectrum of the pigment 

 found in purple bacteria, it is necessary either to prepare an 

 emulsion of the organisms in water, or to extract the colouring 

 matter with a suitable solvent. An extract with chloroform 

 or carbon bisulphide is purple in colour ; with ether or alcohol 

 it is yellow or yellowish-green. 



Engelmann used an aqueous suspension of Bac. photo- 

 metricum and determined the exact amount of absorption at 

 different wave-lengths by the use of Langley's Bolometer.f 



Figures on the following page show the results obtained 

 by him. 



A gives the wave-length of the different colours. 

 E gives the percentage of energy that is transmitted after 

 the light has traversed the coloured fluid. 



* See page 200, footnote. 



t The Bolometer is an instrument used for estimating the total energy 

 contained in any part of the spectrum. As it travels along the spectrum 

 it picks up the energy, with the result that it causes an alteration in the 

 resistance of an electrical wire with which it is in attachment. The amount 

 of alteration in the resistance is measured by a galvanometer. 



