After 3 h. the cotyledons were doubtfully curved towards the
light, and after 7 h. 40 m. from the first exposure, they were all plainly,
though slightly, curved towards the lamp. Now, at this distance of 12 feet,
the light was so obscure that we could not see the seedlings themselves,
nor read the large Roman figures on the white face of a watch, nor see a
pencil line on paper, but could just distinguish a line made with Indian
ink. It is a more surprising fact that no visible shadow was cast by a
pencil held upright on a white card; the seedlings, therefore, were acted
on by a difference in the illumination of their two sides, which the human
eye could not distinguish. On another occasion even a less degree of light
acted, for some cotyledons of Phalaris became slightly curved towards the
same lamp at a distance of 20 feet; at this distance we could not see a
circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on white
paper, though we could just see a dot 3.56 mm. (.14 inch) in diameter; yet
a dot of the former size appears large when seen in the light.
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