hyperbolas, and combinations of them, besides many figures of fanciful forms, are included in the set. When an image of the sun is viewed through these apertures, figures of extraordinary beauty, both of form and colour, are produced; and of each of these many variations may be obtained by placing the eye-glass of the telescope at different distances from the object glass. Many of the figures produced, especially when the telescope is out of focus, might suggest very useful hints to those concerned in designing patterns. Although the phenomena are chiefly of interest to the student of science, in consequence of their bearing on theories of light, yet their beauty and variety render them amusing to all. A few words on the mode of using the apparatus may be of service. (Fig. 318.) [Illustration: Fig. 318. Elliott Brothers' diffraction apparatus.] Choose a very bright day, for then only can the apparatus be used. Place the mirror in the sun, and let the light be reflected on the back of the blackened screen. The lens which is inserted into this screen will then form an exceedingly bright image of the sun. Then at the distance of not less than twelve feet, clamp the telescope to a table in such a position as to view the image thus formed. Put the eccentric cap on the end of the telescope, clean the glass objects carefully, and attach them to the cap so that they may be turned each in order before the telescope. In this manner, all those which consist of a series of figures may be viewed. Then detach the eccentric cap, and replace it by the other. Into it place any of the single objects. In viewing some of the figures, brightness is advantageous--in others, delicacy; in the former case, let the lens of long focus be inserted in the screen--in the latter case, that of shorter focus. In every case, let the phenomena be observed not only when the telescope is in focus, but also when the eye-glass is pushed in to various distances.