inches' focal length some twenty feet distant. About half-way a disc of about a quarter-inch diameter, and very smoothly and accurately turned, is suspended by three threads,[1] so that its center is accurately in line with the pinhole and the center of the lens. The field of the lens will now be quite dark, except at the center of the shadow, where a bright point of light is seen.
We shall now attempt to show the analogue of the sound-shadow experiment by means of light waves. The light is 
FIG. 18concentrated on a very narrow slit A (Fig. 18), which may be supposed to act as the source of light waves. Another slit B, about an inch wide, is placed at a distance of about eight feet, and beyond this a screen C receives the light which has passed through B. The borders bb of the shadow of the slit B are quite sharply defined (though a very slight bending of the light around the edges may be observed by means of a lens focused on b). But if the slit be made narrow, as at B', the sharp boundary which should appear at cc is diffuse and colored, the light being bent into the geometrical shadow as indicated by the dotted lines. The narrower the second slit is made, the wider and more diffuse will be the image on the screen; that is to say, the greater will be the amount of bending into the shadow. An interesting variation of the experiment is made by using two slits instead of the second slit B. In this case, in addition to the
- ↑ The disc may be glued to a piece of optical glass, care being taken that no trace of glue appears beyond the edge of the disc.
