Joseph Plateau also created the phenakistiscope.
ANORTHOSCOPE. An optical toy which distorts figures viewed through it. It consists of two discs, on one of which the figure to be viewed is painted, while in the other there are slits through which the observer looks, as in the Zoetrope. The discs are so arranged as to revolve in opposite directions, and the disc bearing the figures is made transparent, so that it may be seen by holding it up toward the light. The figures are usually so drawn that when viewed by the unaided eye they are unrecognizable, but when placed in the anorthoscope they are restored to their proper shape. The arrangement and results of the toy depend somewhat on the relative velocity of the disks. We will suppose that the disk bearing the slit is made to revolve once, while that with the figure does so four times. Then there must be four slits in the front disk, arranged thus -|-, and, whatever figure may be drawn on the other disk, five distorted figures, all alike, will be seen by looking through the slits. The illustrations on page 14 show the appearance of two designs, first as seen with the naked eye, and then through the slits.
The reason why the toy produces this effect will now be given. First suppose there is only one slit in the front disk, and only a dot, instead of a picture, on the other. Suppose the disk to start with the dot just behind the slit. As the back disk turns four times as fast as the front one, the dot will pass behind the slit four times before they get around into the same position again. Thus the eye will see five dots on the rear disk instead of one. If there are four slits at right angles the result will be the same, for each will pass the dot in the same place as the others. But there cannot be more than four. The same will be true of a large figure as of a dot, but each of the multiplied figures will be shut together like a fan, so as to extend only one-fifth as far around the circle as before. That is, supposing the circle to be divided into 360 degrees, if the picture extended around sixty degrees, it will appear in the anorthoscope to extend over only twelve degrees. This shutting together is a consequence of the rapid movement of the rear disk past the front one. If this reduction in size took place in all directions, the figure would be the same shape, only smaller, but it takes place in only one direction, that is, around the circle, hence the figure is twisted out of shape.
Any figure may be drawn on the disk so that it will appear in its proper shape when viewed through the anorthoscope. Suppose the figure to be that of a card as shown in the illustration. Draw lines from the center of disk through the angles of the card, and others to the points 1, 2, 3, etc., at intervals of any desired number of degrees, say five, as in the plan on page 15. The position of the card should be so arranged that the lines passing through the corners will be multiples of five degrees apart. (The degrees may be laid off with a curved scale, called a protractor, sold by any dealer in drawing materials.) Then draw an equal number of lines from the center, twenty-five degrees apart to the points 1', 2', 3', 4', etc., representing the first lines opened out like a fan. Take any line of the figure, and measure the distance, from the center, of the point where it crossed each of the radiii first drawn, and make a dot on the corresponding new radius at just that distance. For instance, measure the distance from the center to the left-hand corner on the radius drwan to 1, and then lay it off on the radius drawn to 1'. Join all the dots so made by a curved line, and do the same with all the other lines of the figure. Care must be taken that the original figure does not take up more than one-fifth of the disk; otherwise the adjoining figures, as seen in the anorthoscope, will overlap.
Anorthoscopes can be made which will multiply the figure seen as many times as desired, shutting it together to a corresponding degree. The number of figures seen is always one greater than the number of revolutions the back disk makes while the front one is going around once, and the number of slits, always one less than the number of figures, must be disposed at equal distances around the disk. Thus, if it makes eight to the front disk's one, nine figures will be seen, each of which reaches only one-ninth as far around the circle as the original. In this case there must be eight slits.
The anorthoscope may be made to work in many other ways besides the one described here. If the disks revolve in the same direction the number of revolutions can be so adjusted as to combine several figures into one, instead of expanding one into several. By slightly varying these figures an effect is obtained like that of the ZOETROPE.
The anorthoscope is not commonly sold at toy stores. The disks can easily be made as above described, but it is more difficult to make the disks revolve at exactly the proper rate This can be effected by means of cog-wheels arranged as shown in the illustration. If the number of cogs on the larger of the two parallel wheels be four times that on the smaller, the latter will revolve four times as fast. The number on the crank-wheel is immaterial. The arrangement can be made at any machine shop.
The anorthoscope is the invention of Prof. Plateau, a Belgian scientist. The name is from the Greek anorthos, crooked, and skopein, to see.