236 



NATURE OF PHOTOSENSORY PROCESS 



of the apparatus is shown in Fig. 4. The top of the table B is painted 

 black except for a white streak parallel to the long axis of the light 

 beam. The white streak is a background on which to view the move- 

 ment of the siphon. In addition the streak is graduated in centi- 

 meters marking distances from the center of the source of illumina- 

 tion. In this way the siphon of the animal may be placed at exactly 

 the desired distance from the Hght, where it will receive the proper 

 exposure from the shutter. 



comparffvenf - Y 



^ (Animal in dish 

 o-f seaivafer 



Fig. 4. Arrangement of apparatus in the dark room. The air bulb attached 

 to the spring release clip of the shutter by means of rubber and glass tubings is 

 not shown in the figure. With the help of this bulb and tubing, an exposure can 

 ibe effected from any position at the work table. 



rv. 



The first experiments with which I undertook to test the photo- 

 chemical nature of the Hght sensitivity were concerned with the rela- 

 tion between the exposure time and the minimum intensity necessary 

 to produce a response. Since the discovery of the Reciprocity Law 

 by Bunsen and Roscoe in 1862, its vahdity has been demonstrated 

 for a variety of purely photochemical reactions. It obtains whenever 

 the quantity of photic energy necessary to produce a given chemical 

 effect is constant, whether the intensity is low and the exposure long, 

 or the reverse. There are complex reactions, partly photochemical 



