but if dots had been made every 1 or 2 minutes, the lines would have been
more curvilinear, as occurred when radicles were allowed to trace their own
courses on smoked glass-plates. To make the dots accurately was the sole
difficulty, and required some practice. Nor could this be done quite
accurately, when the movement was much magnified, such as 30 times and
upwards; yet even in this case the general course may be trusted. To test
the accuracy of the above method of observation, a filament was fixed to an

* These terms are used in the sense given them by De Vries, 'Würzburg
Arbeiten,' Heft ii 1872, p. 252.

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inanimate object which was made to slide along a straight edge and dots
were repeatedly made on a glass-plate; when these were joined, the result
ought to have been a perfectly straight line, and the line was very nearly
straight. It may be added that when the dot on the card was placed
half-an-inch below or behind the bead of sealing-wax, and when the
glass-plate (supposing it to have been properly curved) stood at a distance
of 7 inches in front (a common distance), then the tracing represented the
movement of the bead magnified 15 times.

Whenever a great increase of the movement was not required, another, and in
some respects better, method of observation was followed. This consisted in
fixing two minute triangles of thin paper, about 1/20 inch in height, to
the two ends of the attached glass filament; and when their tips were
brought into a line so that they covered one another, dots were made as
before on the glass-plate. If we suppose the glass-plate to stand at a
distance of seven inches from the end of the shoot bearing the filament,
the dots when joined, will give nearly the same figure as if a filament
seven inches long, dipped in ink, had been fixed to the moving shoot, and