326 Brown.—Studies in the Physiology of Parasitism . /. 
coherence has been so far reduced as to be imperceptible when tested by 
the hand—that is, when the disc is pulled from opposite sides it separates 
without perceptible resistance. The exact position of this point in the 
series of changes in the tissue brought about by the extract obviously 
depends on the ‘ stimulus threshold ’ for resistance to pull of the particular 
observer. This limiting value varies with the same observer from time to 
time, and thus a varying personal equation is introduced. In cases where 
two extracts of approximately equal strength are being tested, it is obvious 
that if the determinations are made within a short time of each other, the 
personal variation is small. When two extracts of widely different activities 
are being compared, it may happen that the end-points are reached at 
widely different times, in which case the personal factor may be different in 
the two determinations. It is, however, plain that in such a case the 
necessity for accurately determining the end-point is diminished. 
The accuracy of determination of the end-point obtainable in practice 
may be gauged from the following figures. In the case of an experiment 
which would be set down as finished in fifty minutes, it is quite possible to 
convince oneself that a definite degree of coherence is perceptible after 
forty-five minutes; in other words, it is quite feasible in an action lasting 
fifty minutes to determine the end-point to within five minutes. This gives 
an error limit of about 10 per cent. Furthermore, when discs from the 
same region of an approximately uniform tissue are tested with the same 
extract, their end-points are found to show agreement to within 10 per cent. 
This figure then represents roughly the limit of variation which ceases to 
be significant. In the following it will be seen that the differences of 
activity noted are as a rule much greater than this, and no conclusions are 
drawn from observed differences in activity in which it was not perfectly 
certain that the differences observed lay far beyond the limits of this 
observational error of 10 per cent. 
(iv) The varying nature of the actual substrate. There is wide 
variation in sensitiveness of the different potatoes, turnips, &c., employed. 
Thus with the same extract a variation of 100 per cent, has been 
observed in different potatoes. The same has been shown to apply to 
turnips. This consideration renders the comparison of results of different 
series of experiments difficult, and in practice this was carried out by com¬ 
paring each with a standard. This is furnished by the standard extract 
above described, a common stock of spore material being kept for purposes 
of this day-by-day comparison. When, however, the comparison of two 
sets of experiments was considered to be of critical importance, it was 
always possible to carry out the observations side by side on the same 
substrate. 1 
1 It might be thought feasible to cut from the same potato a large number of discs, which 
could then be kept in presence of an antiseptic and used for standardizing purposes. This method 
