598 BIOLOGICAL EFFECTS OF RADIATION 



the same effect as continuous light, provided the frequency is sufficiently 

 high. It is therefore evident without observation that the Bunsen- 

 Roscoe law holds in some respects for every system and that its estab- 

 lishment is of importance only when it is specifically correlated with flash 

 frequency, i.e., the duration of the exposure period, and with the intensity 

 of the light (Mast, 148). 



Daphnia, as stated above, usually orients with its back toward the 

 source of light. If the direction of illumination is changed, it turns 

 directly from the light until it is again oriented. If it is held so that 

 the body cannot turn, the eye turns so as to assume as nearly as possible 

 its former orientation (Radl, 189). 



Clarke (37, page 184) comes to the following conclusions: "Orienta- 

 tion is brought about by the differential stimulation by the light of the 

 bilaterally symmetrical organ of light reception. The consequence is 

 that differential changes in the posture of the two swimming appendages 

 on the two sides of the body are produced. As a result the animal is 

 turned until equal amounts of light are received by both sides of the eye 

 and orientation is complete." It "conforms to the general theory of 

 phototropic orientation," meaning, I assume, Loeb's tonus theory. 



He presents no evidence in support of these conclusions. It may 

 well be that orientation is the result of change in the direction of the 

 stroke of the antennae correlated with the location of the stimulus in the 

 eye, rather than to "differential changes in the posture" of the two 

 antennae correlated with the relation in the amount of light received 

 by opposite sides of the eye. Detailed observations on the posture and 

 the action of the antennae would doubtless yield results which would 

 have important bearings on this problem. Moreover, according to 

 Ewald (62), when the eye is oriented in light from two sources, the two 

 sides are equally illuminated only if the light received from the two 

 sources is equal. If this is true it is obvious that when Daphnia is 

 oriented under natural conditions "both sides of the eye" rarely ff ever 

 "receive equal amounts of light." 



Ewald (62) maintains that intermittent light has per given quantity 

 the same effect as continuous light on the orientation of the eye and on 

 the position of aggregation of free specimens, and that this effect is 

 independent of flash frequency down to 1 per sec. and consequently that 

 the process of orientation and the Bewegungsreflexe are independent. 

 He postulates that the former is due to stimulation of the lateral, and 

 the latter to stimulation of the anterior ommatidia in the eye, but he 

 presents no evidence in favor of this postulation. 



Clarke (37) and Clarke and Wolf (38) observed that strychnine makes 

 photonegative specimens photopositive without causing any change in 

 orientation and they maintain that this shows that the two processes are 

 distinct. They consequently support Ewald in the contention that the 



