EFFECT OF CRUDE OIL POLLUTION ON OYSTERS 
195 
turning on of the battery and small fluctuations occurred every time the current was 
turned off and on. To avoid this difficulty a second Mazda bulb (Z) of the same 
candle power was introduced into the circuit. By means of a double switch (S) the 
current from the storage battery could be turned either through the bulb mounted 
in the box or through the second one located outside. During the observations the 
current was continuously passing through the spare bulb except during brief moments 
when measurements of the samples were made. By this arrangement the overheat- 
ing of the box and difficulties due to the fluctuations in the voltage of the storage 
battery were overcome. The battery was kept well charged. 
Figure 11. — Photoelectric set-up used for counting number of cells in Nitzschia culture. Ph, photoelectric cell; A, microammeter; 
C, glass container with fascil bottom made of optical glass; L and Li, electric bulbs; Ql, ground glass; S, double key switch; 
and B, storage battery. 
For the standardization of the apparatus the bulb was so adjusted that the light 
passing through the container filled with 50 cc of filtered sea water caused the needle 
deflection of the microammeter to stand at 30. This represented the zero point of the 
calibration curve. Other points were obtained by making readings with various con- 
centrations of Nitzschia and counting by means of Sedgwick-Rafter chamber the 
corresponding number of cells. Altogether 35 samples were counted and from the 
data obtained, a curve was plotted which permitted the conversion of microammeter 
readings into number of diatoms per cubic centimeter. 
Before withdrawing a sample for measurement each culture flask was thoroughly 
shaken for 2 minutes then 50 cc were taken by means of a certified volumetric pipette, 
and poured into container c. Care was taken to avoid air bubbles, the presence of 
which affected the ammeter readings. Upon measurement, which required about 
30 seconds, the sample was returned to the same flask and both the container and 
pipette carefully washed in sterile distilled water and dried. Because of the sim- 
plicity and quickness of the operation it was possible to experiment with several dozens 
of flasks simultaneously. 
