The experiment is made more simple by taking a hat pin with a
conspicuous head, holding it firmly in a horizontal position, and
putting a cork on the point. Looking at it in semi-darkness, one
seems to see sometimes the head of the pin, sometimes the point
towards him, when he knows which direction is right. The inversion
will be continued as soon as one observes fixedly a point at the
side. Here it is a question of the perception of depth or
distance; and this is the same in the case of the rotating balls;
the direction of seeming revolution depends on which one of them
one considers to be the front one and which the rear one.
From the foregoing the following conclusion may be reached: When,
in the case of a perception remitting two appearances, one fixedly
observes one of these and then permits or causes change in the
sharpness of the image on the retina, the other appearance asserts
itself.
** Steam Engine Made from Gas Pipe and Fittings [184]
Almost all the material used in the construction' of the parts for
the small steam engine illustrated herewith was made from gas pipe
and fittings.
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