658 LATENT PERIOD OF MYA 



II. 



During the exposure of Mya to light, a photosensitive material is 

 decomposed into its two precursors. If these precursor substances 

 are in any way concerned with the nature of the latent period, there 

 should exist some relation between the quantities in which they are 

 produced and the duration of the latent period. Experiments were 

 therefore performed to determine the influence of the exposure period 

 on the length of the latent period. 



The reaction time of Mya under given conditions is quite a con- 

 stant quantity, and is well over 1 second in length. Any signifi- 

 cant changes in its duration can, therefore, be measured by means 

 of an ordinary stop-watch. On the other hand, the exposure periods 

 necessary to produce a response are very small, and must be measured 

 in a much finer manner. This was done by means of a camera shutter 

 loaned to me by Dr. A. C. Redfield, to whom I am greatly indebted. 



Seven exposures were selected for use. They were calibrated by 

 photographing a small white spot moving in a circle at a definitely 

 known rate in front of a dead black surface. The photographs were 

 made at noon of a brilliantly sunht day, using the largest stop of a 

 lens about 4 cm. in diameter. Therefore the angle subtended by the 

 streak image on the photographic plate may be considered an accur- 

 ate measure of the duration of the exposure. The seven exposure 

 periods were found to cover a range from 0.015 to 0.257 second; their 

 values are given in Table I. 



TABLE I. 



Duration of Exposures Obtained from the Camera Shutter Employed in 

 These Experiments. 



