12
G. JE. Seashore,
3. Visual memory of size causes a less intense illusion of weight
than that which is produced by looking at the blocks while lifting*
4. Size has no influence on the perception of weight when the
observer has no knowledge of it.
fourth series of experiments : Dependence of the illusion of weight
upon the senses by which knowledge of size is acquired.
In all the foregoing experiments the perception of difference in
size was purely visual. The next problem was to settle the question :
Does the illusion of weight vary with the different senses by which
the image of size is produced ?
It is impossible to draw a strict line of demarcation between the
different sensory elements which, besides sight, unite in building up
a percept of size, because they generally cooperate, and their effects
fuse. For the purpose of comparison, I aimed to get a measure of
the illusion of weight due to size when knowledge of size was
acquired through each of the following channels predominatingly :
(1) muscle sense, (2) touch, (3) sight, (4) muscle sense, touch and
sight.
These conditions were approximately attained by the following
respective methods of procedure.
1. Without having seen the blocks, the observer seated himself on
a high stool behind a screen, in such a position as was occupied when
he stood by the tray and looked at the apparatus, so that his arm and
hand would be comfortably adjusted over, the tray, on the other side
of the screen. The blocks were then handed him in the same order
as before, but placed on end on the tray, so that by dropping his
thumb and fingers around a block the observer included it in his
grasp, holding it by the circumference ; he thus acquired a knowl¬
edge of its size mainly by the muscle sense in the fingers.
2. Seated in the same position, after a brief rest, the observer held
out his hand, not resting it on anything, and the blocks were placed
on his flat palm, one at a time. An image of the height (length) of
the block was inevitably transmitted from the experience in the first
set, but the proportional size was here estimated chiefly by the
area of touch or pressure.
3. The method of estimating comparative size by direct sight, as
pursued in the previous experiments, eliminates all other sensory
elements than sight.