IRRITABILITY : INFLUENCE OF LIGHT 211 



Fig. 57- The cilium then revolves round the hinder part of 

 the body and propels the spirillum in the reverse direction. 



[b) Reaction to Light.- — The reaction following the exposure 

 of an organism to light of a given intensity depends on the in- 

 tensity of the light to which the organism was previously ex- 

 posed. There may be, for example, a reaction when the liglit 

 intensity is increased from 18 units to 20 units (units arbitrarily 

 chosen), but not from 40 to 42, although the difference in inten- 

 sity, namely, 2 units, is the same. Before a reaction due to a 

 change of intensity occurs, the new intensity must be a certain 

 percentage greater than the original intensity, and so the greater 

 the original intensity the greater the number of units which nfiust 

 be added to raise the percentage to the required point. The 

 susceptibility of the human eye to a change in the intensity of 



Fig. 57. 



light is formulated in Weber's Law, which states that before 

 the eye can appreciate a change in the intensity, the increment 

 of light units must bear a certain ratio to the original intensity, 

 and this ratio is independent of the value of the intensity. 

 Weber's Law does not hold for the reactions of Thiospirillimi 

 jenense to light, for whilst in some cases an increase of 10 per 

 cent, on the original intensity was required to produce the effect, 

 in other cases as much as 25 per cent, was necessary. Again, 

 according to Buder, the changes in the direction of move- 

 ment in response to changes in the intensity of light are not 

 invariably as sudden as was claimed by Engelmann. The 

 length of time required for the completion of the reaction 

 depended on the velocity of the organism at the moment, the 

 length of time taken by the light to change its intensity, and 

 on other factors, so that under some conditions quite an 



14 * 



