74 
Proceedings of the Royal Society of Edinburgh. [Sess. 
the mental working capacity for that period — to be ascertained, but also 
by a simple device the number of units per minute throughout the period. 
By plotting the number per minute of these mental units upon a curve, 
the progress of the mental working capacity throughout the period is 
objectively shown, so that we can thus demonstrate mental work in a 
manner analogous to that by which we can through the use of the ergograph 
or the dynamometer demonstrate muscular work. The mode of using the 
test is as follows The patient is required to add the digits as rapidly 
as possible in pairs ; each investigation lasts a fixed time, usually ten or 
fifteen minutes. At the end of every minute the examining physician calls 
“ Stroke,” and the patient then makes a line under the last digit he has 
written : the number of units between any two successive lines shows the 
rate of work in that particular minute. To minimise time and effort in 
writing, to make the test an index of the intellectual, not of the muscular 
processes involved, only the unit of each sum is written; thus 6 + 7 = 3, 
9 + 8 = 7. To economise space the method of continuous addition is always 
employed, if the patient be sufficiently intelligent to grasp it. Excepting 
the first and last figures in the columns, each is used twice, once added 
to the digit which precedes it and once to that which follows it. 
8 
1 8 + 3 = 11 
3 
8 3 + 5=8 
5 
2 5 + 7 = 12 
7 
6 7+9 = 16 ( 2 ) 
9 
Besides the total number of units of work done affording an index 
to the mental capacity, the accuracy of the work done affords an index 
to the condition of the fixed associations. During the performance of the 
test, as might be expected, errors were frequent, some in the form of faulty 
addition of the right units, some in the correct addition of the wrong units, 
whilst others were made but subsequently corrected by the person doing 
the test. The nature of these errors was interesting. Thus it was found 
that the same error was constantly repeated by the same person, e.g. two 
sevens were persistently called 15 instead of 14, or nine and seven 17 
instead of 16, etc. In this paper I do not propose to deal with the errors, 
but only with the question of the mental capacity, which is not influenced 
by errors such as I have mentioned. 
The object of this research was to ascertain whether or not any definite 
characteristic of the mental working capacity could be detected in the 
