IRRITABILITY : INFLUENCE OF LIGHT 197 



according to the suddenness of the change from Hght to dark- 

 ness. Again all the individuals were not affected to the same 

 extent, and some failed to respond altogether. Similar 

 effects were obtained by causing the organism to traverse 

 from one colour to another. If the intensity of light in the 

 spectrum is great, the organism travels from one end of the 

 spectrum to the other with undiminished vigour, but if the slit 

 be made very narrow, shock movements are observed in 

 passing from one colour to another. They were observed in 

 the passage from beyond infra-red to infra-red ; from infra-red 

 to red ; from yellow to red ; from yellow to green ; from green 

 to blue ; from blue to violet. 



With one exception the reactions occurred when the 

 organisms passed from a colour of greater to one of lesser wave- 

 length. Engelmann did not apparently observe the effect 

 of a passage from a colour at one end of the spectrum direct 

 to one at the opposite end, but he noted that shocks were 

 observed in passing from any colour to the dark, and that the 

 shock was not so pronounced when the organism entered the 

 dark from either extremity of the spectrum. 



Shock Movements a form of Preservation : 

 Valve Action. 



Engelmann maintained that the shock movements exercised 

 a self-preserving effect on the organisms, as their tendency was 

 to prevent their passage into the 

 dark. Thus an individual moving in 

 the dark along the line ah (see Fig. 

 53) experiences no shock on reaching 

 the light, but when it crosses the 

 area of light and enters the dark 

 zone on the other side a shock 

 movement occurs which brings the 

 individual back once more to the pj^ _^ 



lighted region. The influence of 



light will now operate to prevent it once more taking 

 up the path ah which would lead it into the dark. If the 

 Hght is strong enough the reversed movement caused by the 



