232 



NATURE OF PHOTOSENSORY PROCESS 



In order to demonstrate graphically this division of the reaction 

 time, I used an optical recording de\ice.' The animal is placed \er- 

 tically in sand in a glass trough filled with sea water. A fine thread 

 connects its siphon tip with a light heart lever as near the axle as 

 possible. To the opi)ositc side of the axle is attached a small mirror 

 which rcllccls a beam of light into the slit of the recording camera. 



Fig. 2. Optical record of slimulatioii and response, showing the short exposure 

 period and the long latent period. The white streak running the length of the 

 figure represents the movement of a mirror attached to the siphon tip of Myn. 

 The white spot and vertical streak at the left record the duration of the exposure 

 (0.07 second). The siphon retracts nearly 2 seconds after the exposure. The 

 slight irregularity in the siphon record immediately following the exposure is due 

 to an accidental vibration of the delicately balanced mirror caused by the move- 

 ment of the shutter. The tuning-fork marks 0.02 second. 



A contraction of the siphon will be shown by an upward movement 

 of the reflected beam. Passing through a shutter is another beam of 

 light. This beam plays on the expanded siphon, and beyond that, 

 directly on the camera sht. On opening the shutter, the beam will 



^ Dr. D. J. Edwards was kind enough to let me use his optical recording appa- 

 ratus, and to give me much of his time and help in the making of records, for all 

 of which I wish to express my gratitude. 



