SELIG HECHT 235 



rod R is attached strong twine, which on the left connects with a 

 knob K, and to the right leads to the projecting bar M. Therefore 

 an upward movement of the bar results in the sHding of the cardboard 

 across the length of the shutter. 



The motion of the projecting bar is due to the release of the heavy 

 brass wire spring NN' wound several times around the screw W. 

 The tension of the spring and therefore the speed of the shutter may 

 be varied by placing the arm N' of the spring against the appropriate 

 raised screw head 1, 2, 3, or 4. The caKbration of this movement is 

 accomplished by recording the excursion of the tip of the bar M on a 

 rapidly moving kymograph. Several such records gave identical re- 

 sults. A few hundredths of a second after being released, the shutter 

 sKde moves with a constant velocity, which in these experiments is 

 67.5 cm. per second. The exposure may therefore be computed from 

 the width of the aperture in the sHde, by multiplying the width by 

 the fraction of a second during which the sHde moves 1 cm. (= 0.0148 

 second) . The front and rear boards of the shutter have raised grooves 

 V for the insertion of cards having different sized stationary openings 

 O. In this manner, the dimensions of the beam of light may be 

 varied. In these experiments the size of the stationary opening is 

 5 X 35 mm. 



To get the shutter ready for use, the opening is temporarily cov- 

 ered with a black card, and the knob K is pulled until the shutter 

 slide C is brought to the left of the shutter. Here it is caught by the 

 release clip F and held in position. The movement of the shutter 

 slide C of course pulls the bar M down to the top of the shutter 

 against the tension of the spring NN'. The temporary card is then 

 removed, and the shutter is ready for an exposure. At the proper 

 moment the spring clip is released, and at once the spring NN' exerts 

 its action. The bar M is jerked rapidly upward and the shutter 

 sHde C is shot past the opening O, thus effecting the proper exposure. 



When set up, the shutter as a whole is placed in the front wall of 

 a Kght-tight compartment Y (Fig. 4), containing a 250 watt concen- 

 trated-filament Mazda lamp. The center of the beam through the 

 opening O is a few millimeters above the level of a long table B. This 

 is so that the beam will be exactly centered on the siphon of an animal 

 in a dish of sea water standing on the table. The general arrangement 



