SELIG HECHT 233 



therefore stimulate the animal, and at the same time record the exact 

 duration of the exposure on the moving film.. The shadow of a vibrat- 

 ing tuning-fork is also focused on the slit, so as to furnish a time record. 

 One of the records obtained in this manner is given in Fig. 2. With 

 an exposure of 0.07 second at this intensity, the retraction of the 

 siphon appears only after a latent period of nearly 2 seconds. A 

 record Hke that of Fig. 2 is striking testimony of the composition of 

 the reaction time. Since it is our purpose to study the photochemical 

 aspect of photic sensitivity, our attention must therefore be devoted 

 to the relatively short exposure period during which the reception of 

 the light takes place. 



III. 



Before taking up the nature or the details of the experiments, it is 

 necessary to describe the apparatus which is used for the accurate 

 control of short exposures. In principle the mechanism is that of a 

 focal plane shutter. Its construction may be described with the help 

 of the three views in Fig. 3. Essentially the shutter consists of a 

 piece of black cardboard C, having a variable aperture A which moves 

 with a definite speed past an opening 0, through which a beam of 

 light is directed. The duration of the exposure depends on the 

 velocity of the cardboard and the size of the aperture. The rest of 

 the apparatus serves merely to control these two factors accurately 

 and easily.^ 



The variable aperture is obtained by using slides S, each having a 

 different sized opening — all slides, however, having the same weight. 

 A slide is placed in the apparatus by sHpping it into the raised grooves 

 G; it is kept in place by them with the help of the raised end-piece P. 

 The front of the shutter may be easily removed for the exchange of 

 sKdes by turning the thumb screws T. The cardboard C which holds 

 the slide is attached at the bottom to a cylindrical wooden rod R. 

 As it moves across the field the cardboard slides in a simple groove at 

 the top of the shutter, and the wooden rod sHdes in a metal tube Z 



^ I made this apparatus by converting a focal plane shutter originally con- 

 structed by Mr. J. G. Hubbard. I take this opportunity of thanking Mr. Hub- 

 bard for much in the way of advice and material assistance in the building of 

 apparatus used in this work. 



