Anecdotes find their way into print, from time to time, of persons
whose visual memory is so clear and sharp as to present mental
pictures that may be scrutinised with nearly as much ease and
prolonged attention as if they were real objects. I became
interested in the subject and made a rather extensive inquiry into
the mode of visual presentation in different persons, so far as
could be gathered from their respective statements. It seemed to me
that the results might illustrate the essential differences between
the mental operations of different men, that they might give some
clue to the origin of visions, and that the course of the inquiry
might reveal some previously unnoticed facts. It has done all this
more or less, and I will explain the results in the present and in
the three following chapters.

It is not necessary to trouble the reader with my earlier tentative
steps to find out what I desired to learn. After the inquiry had
been fairly started it took the form of submitting a certain number
of printed questions to a large number of persons (see Appendix E).
There is hardly any more difficult task than that of framing
questions which are not likely to be misunderstood, which admit of
easy reply, and which cover the ground of inquiry. I did my best in
these respects, without forgetting the most important part of
all--namely, to tempt my correspondents to write freely in fuller
explanation of their replies, and on cognate topics as well. These
separate letters have proved more instructive and interesting by far
than the replies to the set questions.

The first group of the rather long series of queries related to the
illumination, definition, and colouring of the mental image, and
were framed thus:--