MODES OF MEASURING MINUTE INTERVALS OF TIME. 111 
ever form, they may, in different individuals, or at different times in the same individual, 
be presented to the intellect; and in no case is thought likely to be more rapid than in 
proceeding from number to number, so as to consider each individually, and not bring 
them together in groups or systems. Now if any one endeavours to put a series of them 
through the mind, under this restriction of their being separate and individual as mental 
conceptions, he will probably find that he cannot do so faster than his changes in sensa- 
tion occur. He will find that, in regard to impulses of sound, he cannot number accu- 
rately those which the ear perceives to be distinct. If they amount, for instance, to 12 in 
the second, he will not, by the mind’s effort only, ascertain confidently that such is the 
fact. If the impulses be reduced to 10 in the second, which would correspond with the 
limit of duration as to time in regard to the eye’s sensations, counting of them with confi- 
dent accuracy will be found to be difficult. It may be presumed that no one will, even 
with long continued trial and practice, be able to go beyond this. We may on the whole, 
therefore, conclude that while changes of sensation do not occur oftener, than at the rate 
of one in {, of a second, changes of thought, or of will, require about double the time of 
such perceptions. ‘Thus there are insuperable limits to accuracy of observation, through , 
means of the senses and the will alone. 
Human physiology, therefore, comes here in contact with practical astronomy. In the 
observations of that science the circumstances of this kind which influence their accuracy 
may be complicated to a considerable degree. There may be acts of will, of judgment, 
and of sensation, all occupying time; and capable of variation in that respect in corre- 
spondence with training, habit, or constitutional peculiarities. Two observers watching 
the same phenomenon, such as the meridian passage of a star, will, though in circum- 
stances which, in respect to the phenomenon itself, are identical, differ in their estimate 
of the time of its occurrence. This difference may originate from either or from both of 
two classes of incidents. There may be a difference in absolute time, in the completion 
of the contact to them individually, or in the sensation of the phenomenon; or there may 
be a difference in the accessary circumstances, by which the sensation of the phenomenon 
is referred to a known instant in time. It is probable that individuals differ in regard to 
both these classes of incidents; but that difference will be more conspicuous and important 
in regard to the second rather than the first. Differences in respect to the sensation of 
the phenomenon, though probably the less important, are more completely beyond control 
by any helps or modifications; and where such exist permanently, they will constitute the 
proper personal equations of individual observers, admitting of no modification but that of 
being, when ascertained, kept by training and habit, in a condition as uniform as possible 
in amount. 
