Simple Huyghens' Apparatus for the Optical Lantern

IN his excellent handbook on experimental optics, ``Light,'' Mr. Lewis Wright describes and figures the well-known double-image phenomena to be observed when an ink-dot is viewed through two superposed rhombs of calc-spar. He does not suggest, and I have not seen elsewhere the suggestion, that the experiment is readily adapted to lantern projection. This, however, is the case, and when so projected this experiment is more useful to the demonstrator than that of the double-image prisms, commonly known as Huyghens' experiment, since the apparatus is more simple. There is no colour correction to explain-nothing to occupy the mind of the student but the action of the spar.