174 Trans. Acad. Sci. of St. Louis 
number increased to 17. At 1:50, after the light had been 
played upon them for twenty minutes, 25 moths were counted 
at the light side. When so many were moving together, it was 
hard to tell just how long each moth continued its activity, but 
in this experiment ten minutes seem to be the limit for any 
one individual. Thus we see 71 per cent of this population 
reacting to mild light rays at a time of day midway between 
the two normal periods of darkness. 
Exp. 18. This is a continuation of Exp. 17, but made with a 
purpose of getting additional data on the reactions of moths to 
light of greater intensity. 
These same moths, 35 in number, were placed at the south 
end of the cage and kept in darknesss for 25 minutes. Then 
the curtain was lifted and a 40-watt lamp was pressed against 
the glass at the north end. Now the intensity of the light from 
this lamp was intermediate between the two extremes previously 
used, a 75-watt mazda lamp and the subdued light of the room. 
This being the ease, we should expect the number that respond 
to be intermediate between the numbers that reacted to the two 
extremes. 
During the first five minutes, four moths flew to the lighted 
end, but after a few seconds three of them retired again to the 
dark side and one alone continued its activity. Ten minutes 
after the curtain was lifted, 8 were fluttering at the light; their 
movements gradually became less intense, while slowly a few 
others came, until at the end of 35 minutes, 12 were at rest at 
the light end of the cage. This is 33 per cent of the total, which 
responded to the light of a 40-watt lamp, while less than 8 per 
cent reacted to the strong light of a 75-watt lamp, and 71 per 
cent responded to the diffused or dim light of the room. How 
gratifying it is to find our anticipations fully realized; the pro- 
portion of moths responding to the light of intermediate m- 
tensity was just about midway between the proportion respond- 
ing to either extreme of light! It is also evident that the time 
required to induce the reaction is increased with the intense 
direct light. In 20 minutes we got 25 males to fly at the stim- 
ulus of diffused daylight, whereas in the direct light of a 40- 
watt lamp it required 35 minutes to influence half that number. 
But best of all in this experiment, we prove that the hour of 
the day is no stimulus to action, but light conditions are, Te 
