H 
Macgregor Skene. 
IV. —Relation of the Purple Sulphur Bacteria to Light. 
Engelmann (1888) Winogradsky (1888) and Molisch (1907) 
have investigated in detail the influence of light on the movements 
of Chromatium and other swarming forms: Englemann also 
observed that the growth in light is much better than in darkness. 
On this point Winogradsky is silent. Molisch states that his purple 
bacteria grow only in light, and erects for them a new physiological 
class of organisms, which can assimilate organic carbon only in 
presence of light. 
To investigate this point I carried out several series of cultures 
which gave quite definite results. One set of cultures in the 
following media was placed in the light, another set in a carefully 
darkened cardboard box:— 
1. Lieske’s Solution. 
2. „ ,, + dextrose 0-15%. 
3. ,, ,, + calcium lactate 015%. 
4. ,, ,, + potassium formate 0-15%. 
5. Ammonium sulphate 0-075% + dextrose 0-15%. 
6. ,, „ ,, + calcium lactate 0-15%. 
7. „ ,, ,, + potassium formate 0-15%. 
After two to three weeks the light cultures in Lieske’s solution 
showed a strong development: all the dark cultures were absolutely 
without growth. 
It may be concluded that the sulphur bacteria require light for 
their development. 
Further experiments with a view to studying the effects of the 
different parts of the spectrum were carried out under double bells 
containing the customary solutions, potassium bichromate, and 
copper sulphate -f ammonia. As is well known these solutions 
do not give pure spectra. With bichromate it is possible to shut 
out all the blue rays without too seriously diminishing the light 
intensity: but with the copper-ammonia solution a complete 
exclusion of the red can only be obtained by sacrificing much of 
the blue as well. The blue solution was therefore chosen of a 
strength which permitted of the passage of a considerable quantity 
of red light. 
In red light the bacteria grow very well, though not quite so 
quickly as in daylight. After three weeks (August) the development 
was only moderate, after a month it was very good. In blue light 
on the other hand it is much slower: after three weeks only traces 
of red are to be seen on the sediment: after a month development 
