manufacturer's table of thermistor stock. Resistance curves for various degrees 
centigrade are also furnished. 
Figure 2 illustrates an inexpensive actinometer using a photoconductive cell 
for continuous recording of moonlight, daylight, or low-intensity, crepuscular 
light (Callahan, 1964). The cadmium sulfide cell (CL 505L) is the control 
resistance which, when dark, offers so high a resistance that very little current 
flows in the recorder. As the cell is illuminated by more and more moonlight or 
the rising sun, resistance drops, and higher currents reach the recorder. The 
CL 505L has a light resistance of about 1500 ohms and a dark resistance of over 
a million ohms. The values for the shunt resistors R, and R, were determined by 
using Ohm's law. They are variable trimmer Pesietores so that values for extremely 
low or high light intensity can be inserted in the circuit. This same circuit 
illustrates a use for the photoyoltaic cell- The B10 is a photovoltaic solar cell 
and operates a sensitive Sigma i 5ss 50-ohm relay, which cuts in and opens the 
circuits when excessive sunlight reaches the cell. This prevents damage to the 
recorder, which would draw excessive current under high-intensity light conditions. 
The basic circuit without the protective sun cell device, but with a prism 
over the CdS cell, is shown in fig. 9 as a dew recorder. A low-intensity light 
source (a) is fixed at a far enough distance above the glass prism surface (b) 
so that there is no heating of the prism. As moisture droplets form and disappear 
from the glass surface, light is reflected in all directions, which breaks up the 
base line of light. Figure 10 shows a recording of both moonlight (which took the 
place of a fixed-light source in this test) and the formation of dew along the 
moonlight base line. The recording can be seen rising at about 12:10 a.m. as the 
moon tops some trees. The dew is represented by the broken dotted portion of the 
moon base line, and’ can be seen from 12:35 to 1:05 a.m. and from 1:30 to 2:10 a.m. 
Figure 3 illustrates a further use for such a circuit. A leaf is taped 
above the prism, and a first-instar larva confined on the leaf to feed. A red 
filter may be used to subdue the light. As the larva feeds, the leaf tissue 
decreases and more light reaches the CdS cell, giving both a time and quantity 
line on the recorder. This particular modification is extremely useful for very 
small larvae. 
Figure 4 illustrates the use of a Delco ® 1/ LDR25 high-wattage CdS cell as 
a motor speed control. The 3000-ohm rheostat can be used to turn down the light 
bulb, increasing the resistance of the LDR25, and slowing the motor. Various 
filters could also be used between the light source and the cell and rotated by a 
clock mechanism, causing the motor to slow down or speed up at various set, pre- 
determined times. Almost any load could be substituted for the motor and 
controlled by changing light intensity through the rotating filters. A further 
use of such a high-wattage, CdS cell might be to hook it in series with a cage 
light so that the intensity of an incandescent lamp in the cage would vary 
proportionally to outdoor light shining on the cell. 
I am at present constructing flight-activity cages in which the light is 
continuously and gradually dimmed as the sun goes down. This enables me to 
compare continuously varying light as it is found in nature, with the on-off 
light situations that are usual in the study of diapause, et cetera. I believe 
that the on-off light situation subjects the adults to a "light shock" phenomenon 
that affects flight patterns and behavior of the adult earworm moth. It is only 
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