52 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
now closed, an induction shock was thrown into 
the tissue at the same instant that the electro- 
magnet writer recorded the fact, by altering its 
position on the cylinder. Again, as soon as the 
paralyzed Medusa responded to the induction shock, 
the radii of the vacant segment were drawn apart, 
and in this way a curve was obtained by the other 
writer on the rotating cylinder. Now, by after- 
wards dropping a perpendicular line from the point 
at which the electro-magnet writer changed its 
position, to the parallel line made by the other 
writer, and then measuring the distance between 
the point of contact and the point on the last- 
mentioned line on which the curve began, the period 
of latent stimulation was determined. A glance at 
Figs. 3 and 4 (p. 55) will render this description 
clear to any one who is not already acquainted with 
the method, when it is stated that the upper line 
is a record of the movements of the electro-magnet 
writer, and the lower line that of the movements of 
the other writer. It will be observed that the 
point @ in the upper line marks the point at which 
the induction shock was thrown in; so that by 
first producing the perpendicular till it meets the 
lower line at b, and then measuring the distance 
between the point 6 and the point c, at which 
the curve in the lower line first begins, the latent 
period (bc) is determined—the time occupied by 
the rotation of the cylinder from 6 to ¢ being 
known. 
Summation of Stimuli.—In this way I have 
been able to ascertain the period of latent stimula- 
