MODES OF MEASURING MINUTE INTERVALS OF TIME. 113 
ductions. There is the sensation process of apprehending the fact of contact, and there 
is the complex process of will and of muscular effort, in giving the signal. All these three 
processes ought to be absolutely co-instantaneous. This, however, as we have seen, is im- 
possible; for the three must be historically consecutive; and our only resource to secure 
accuracy must be to render the intervals of time regular in extent, and ascertained in 
quantity. The sensation process, or the apprehension, in the mind, of the contact or bi- 
section will, in this case, differ somewhat in its circumstances from those which character- 
ized it under former modes of observation. When the observer was counting the second- 
beats of a clock, he could retain mutually an apprehension of the space passed over by the 
star during the interval of the two consecutive beats, between which the contact or bisec- 
tion happened, and of the relative size of the divisions before and after into which the 
wire, or the star in contact with it, divided that space; or, in counting minute portions of 
a second, he might both count onwards from the preceding beat to the contact, and thence 
onwards afresh to the subsequent one. In such ways he might get the phenomenon po- 
sited with some degree of accuracy between the two. In the more recent mode of pro- 
ceeding, however, there can be no such resource. The instant of bisection must be the 
instant of signal. If the bisection passes over, the observation is lost, except a time-esti- 
mating process in the mind be employed to correct the instant marked on the time-move- 
ment apparatus, which it would be scarcely suitable to employ except in singular instances. 
The process of observation in general, with all probability, will be, that the observer “wills” 
the muscular movements which are to give the time signal of bisection, before the bisec- 
tion is actually perfect, so as to allow for the interval which these movements need. It 
may be, that such almost unconscious anticipations of events, or mental movements of an 
analogous order, are mainly the cause of those differences in the results of observation, 
which may be termed real or proper personal equations as formerly alluded to. 
Here a principle may be noticed, which is perhaps capable of being introduced with the 
effect of determining the relative perfectness of the bisections, at the instant when the will 
acts to give the signal. This consists, in the definiteness of the last impression on an or- 
gan of sense, when, for a time, no other is permitted to interfere with it. Attention to 
this circumstance explains some interesting phenomena. Nature provides examples of it 
in two modes. When we gaze at any rapidly moving stream or succession of bodies, the 
appearance to the eye, provided its axis of vision remain fixed, will be confused, or be 
merely a formless succession of uniform or slightly varying shades. It is known however 
that if such a stream, as, for example, if a shower of the drops of falling quick-silver in the 
dark be revealed by the quick and instantaneous flash of an electric spark, then will every 
mass in its proper shape appear to be suspended unmoved in the air. A similar effect is 
