believed my eyes. Seeing was believing, I thought. Still more did I
believe my reasoning. It was only slowly that I began to suspect that
the world of fact could be anything different from the clear picture it
made upon my mind.

I realised the inadequacy of the senses first. Into that I will not
enter here. Any proper text book of physiology or psychology will supply
a number of instances of the habitual deceptions of sight and touch and
hearing. I came upon these things in my reading, in the laboratory, with
microscope or telescope, lived with them as constant difficulties. I
will only instance one trifling case of visual deception in order to
lead to my next question. One draws two lines strictly parallel; so

(two horizontal and parallel lines.)

Oblique to them one draws a series of lines; so

(a series of parallel and closely-spaced lines drawn through each
horizontal line, one series (top) sloping to the right, the other
(bottom) to the left)

and instantly the parallelism seems to be disturbed. If the second
figure is presented to any one without sufficient science to understand
this delusion, the impression is created that these lines converge to
the right and diverge to the left. The vision is deceived in its mental
factor and judges wrongly of the thing seen.

In this case we are able to measure the distance of the lines, to find
how the main lines looked before the cross ones were drawn, to bring the
deception up against fact of a different sort and so correct the