I doubt, however, whether “abstract idea” is a correct phrase in many of the cases in which it is used, and whether “cumulative idea” would not be more appropriate.  The ideal faces obtained by the method of composite portraiture appear to have a great deal in common with these so-called abstract ideas.  The composite portraits consist, as was explained, of numerous superimposed pictures, forming a cumulative result in which the features that are common to all the likenesses are clearly seen; those that are common to a few are relatively faint and are more or less overlooked, while those that are peculiar to single individuals leave no sensible trace at all.

This analogy, which I pointed out in a Memoir on Generic Images, [11] has been extended and confirmed by subsequent experience of the process.  One objection to my view was that our so-called generalisations are commonly no more than representative cases, our recollections being apt to be unduly influenced by particular events, and not by the totality of what we have seen; that the reason why some one recollection has prevailed is that the case was sharply defined, or had something unusual about it, or that our frame of mind was at the time of observation susceptible to that particular kind of impression.  I have had exactly the same difficulties with the composites.  If one of the individual portraits has sharp outlines, or if it is unlike the rest, or if the illumination is temporarily strong, it will assert itself unduly in the result.  The cases seem to me exactly analogous.  I get over my photographic difficulty very easily by throwing the sharp portrait a little out of focus, by eliminating such portraits as have exceptional features, and by toning down the illumination to a standard intensity.

[Footnote 11:  “Generic Images,” Proc.  Royal Institute, Friday, April 25, 1879, partly reprinted in the Appendix.]

PSYCHOMETRIC EXPERIMENTS.

When we attempt to trace the first steps in each operation of our minds, we are usually baulked by the difficulty of keeping watch, without embarrassing the freedom of its action.  The difficulty is much more than the common and well-known one of attending to two things at once.  It is especially due to the fact that the elementary operations of the mind are exceedingly faint and evanescent, and that it requires the utmost painstaking to watch them properly.  It would seem impossible to give the required attention to the processes of thought, and yet to think as freely as if the mind had been in no way preoccupied.  The peculiarity of the experiments I am about to describe is that I have succeeded in evading this difficulty.  My method consists in allowing the mind to play freely for a very brief period, until a couple or so of ideas have passed through it, and then, while the traces or echoes of those ideas are still lingering in the brain, to turn the attention upon them with a sudden