igS SULPHUR BACTERIA 



shock can be maintained and the individual remains in the 

 hght. When next it crosses the hghted area and once more 

 reaches the dark zone on the other side a similar shock 

 causes it to reverse again, thereby preventing its passage 

 into the dark zone. The effect of the operation of these 

 factors may be observed by covering a microscope field 

 containing motile purple bacteria with a disc which darkens 

 the whole field except for a restricted region. The organisms 

 tend to collect in this region to the exclusion of the rest of 

 the field. Engclmann compared these shock movements to 

 the action of a valve, presumably because they exercise a 

 certain measure of control on the direction of movement. The 

 analogy is not a happy one, for the essence of valvular action 

 is regulation, and there is no question of regulation of move- 

 ments in the shock phenomena. It is doubtful whether the 

 incidence of these movements is a factor of sufficient importance 

 to affect materially the natural striving of the organism to- 

 wards the light when it is about to pass from the light into 

 the dark. 



MoLiscH ON Shock Movements. 

 Molisch experimented with Rhodospirillum photomelricum* 

 an organism so sensitive that the effect of passing the hand 

 between the light and the mirror produced a shock movement. 

 Molisch was convinced that the colouring matter was closely 

 concerned in the production of these movements, and that in 

 a field of various microorganisms, including the purple bacteria, 

 these could be recognized by their shock movements, even 

 although the colouring matter was not present in sufficient 

 quantity to be perceptible. He confirmed Engelmann's 

 statement that shock movements are more pronounced in an 

 atmosphere containing only a very small quantity of oxygen. 



Distribution of the Purple Bacteria in the various 

 Colours of the Spectrum. 

 Engelmann found that when a spectrum was projected on 

 to the microscope field, the purple bacteria in that field 



* This organism was probably some pleomorphic form of one of the 

 better-known sulphur bacteria. 



