Egon S. Pearson 
31 
between each series than for C. It is mainly by reference to the passing of 
external events, to changes the duration of which we can infer from previous 
experience, that we estimate any but the shortest intervals of time. In the 
counting 'experiment, the second-intervals between each of the 10 counts which 
made up the observation were comparatively short, and the beating of the thumb 
or fingers became almost mechanical ; the interval of course varied but was not 
subject to violent fluctuations. But while most people are able to estimate a 
second interval with fair accuracy, it would need very much practice to estimate a 
10 second interval, and in my case I found it quite impossible to concentrate 
attention for 10 seconds, solely on the passing of time. I soon found myself 
imagining that I saw the seconds' hand of a watch, passing usually from the 
position where 60 is marked on the dial to the 10; but it was not another case of 
counting, for I did not note the passing of each individual second mark, only 
having a vague idea of the position of the 5 second division line. If I tried to 
think of nothing, my thoughts probably wandered on to other subjects, imtil 
I came up with a start, and realising that I had very little idea of how long 
before I had pressed the key to start the observation, pressed it to finish, with 
the greatest uncertainty. To keep attention fixed, it appeared that I must try to 
record the stages of the passage of 10 seconds, and this I was doing vaguely on 
the imaginary clock face, but I must say that the seconds' hand was very re- 
fractory, at times appearing to stojj or even move backwards, and was often so 
slow that I had to close the observation before it reached the 10 second mark. 
I have given the above description at some length in order to shew that there 
was an essential difference between Experiments C and i), which is borne out by 
the figures of the reduction given later in this paper. The observer with tlie key 
sat in a separate room where the beats of the chronogi'apli could not be heard. 
Experiment D was actually carried out in the week previous to C ; before starting, 
a few trials at estimating 10 seconds had been made with a watch, but these were 
not repeated after the commencement. Again, some 10 second counts were made 
with a watch before starting on C, but no comparison with a watch or clock was 
made during the course of the experiment. The measuring up of G and D was 
left until both experiments were completed, so that the chance of some bias 
to the judgment, which occurred in the case of A and B was avoided. 
(c) Experiment E. 
This consists of nine series of readings made with a Zeiss Comparator at the 
Solar Physics Observatory, Cambridge, on photographic plates of the spectrum of 
Nova Aquilae III. The readings were taken in the first j)lace in order to calculate 
the Probable Errors of the measurements of certain types of structure featuring in 
the broad emission bands, and each series consists of readings taken from 51 
consecutive settings on a particular marking, either a maximum or the edge of a 
maximum. Although the number of readings is not sufficient for any great 
weight to be attached to the results, they are, I think, of sufficient interest to be 
included. In the instrument used, the plate to be measured is fixed to a slide. 
