** Optical Illusions [183]

If a person observes fixedly for some time two balls hanging on the end of cords which are in rapid revolution, not rotation, about a vertical axis, the direction of revolution will seem to reverse.  In some experiments two incandescent “pills” of platinum sponge, such as an used for lighting gas-burners, were hung in tiny aluminum bells from a mica vane wheel which was turned constantly and rapidly in one direction by hot air from a gas flame to keep the platinum in a glow.  The inversion and reversion did not take place, as one might suppose, at the will of the observer, but was compulsory and followed regular rules.  If the observer watches the rotating objects from the side, or from above or from below, the inversion takes place against his will; the condition being that the image on the retina shall be eccentric.  It takes place also, however, with a change in the convergence of the optical axes, whether they are parallel to each other or more convergent.  Also when the image on the retina is made less distinct by the use of a convex or concave lens, the revolution seems to reverse; further, in the case of a nearsighted person, when he removes his spectacles,

[Illustration:  Illusions Shown by Revolving Platinum Sponge “Pills” and Hat Pins]

inversion results every time that the image on the retina is not sharp.  But even a change in the degree of indistinctness causes inversion.

The cause of this optical illusion is the same where the wings of windmills are observed in the twilight as a silhouette.  It is then not a question of which is the front or the back of the wheel, but whether one of the wings or the other comes towards the observer.  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.  The cylinder consists of a 3-in. tee, the third opening being threaded and filled with a cast-iron plug turned to such a depth that when the interior was bored out on a lathe the bottom of the plug bored to the same radius as the other part of the tee.  The outside end of the plug extended about 1/4-in. and the surface was made smooth for the valve seat.  A flat slide valve was used.