Diffraction Experiments. 127 



a candle-nauie, we see, with the particular ruled glass em- 

 ployed, a luminous coloured spindle about eight feet long. 



For the nest experiment we take a piece of blackened card 

 and cut a thin slit in it. We hold this in one hand near the 

 candle, and look at the light through the ruled glass, with the 

 lines and slit both vertical. We have now an exquisite ribbon, 

 composed of dark spaces and coloured light spaces arranged as 

 before. If we keep the ruled lines in this position, the diffraction 

 light ribbon will be at right angles to them, and as we revolve 

 the slit, the ribbon will seem to turn so as to present itself alter- 

 nately edgewise and full front. If we keep the slit still, and 

 revolve the ruled lines, the ribbon of parti-coloured light will ap- 

 pear to experience two motions : one round and round, like a 

 windmill arm, and the other the kind of revolution just described, 

 giving us sometimes an edge view and sometimes a full view 

 of the coloured stripes. 



Mr. Slack has devised a very simple apparatus, the 

 " Diffraction Kaleidoscope," manufactured by Messrs. Home and 

 Thornthwaite, which will enable a number of brilliant experi- 

 ments of this kind (including many originally performed by 

 Frauenhofer) to be made at the very moderate cost of pur- 

 chasing the instrument, and without any trouble. 



THE DIFEBACTION KALEIDOSCOPE. 



The Diffraction Kaleidoscope consists of a tube, at one end 

 of which is a revolving eye-piece, and at the other a revolving 

 holder of discs of blackened card having certain apertures 

 through which light is permitted to enter. The experiments just 

 mentioned may be best performed by this instrument. A card 

 disc with one pin-hole in the centre is placed in the disc holder, 

 and an eye-piece containing a " grating" or ruled glass is 

 slipped on to the opposite end of the tube. On looking through 

 this at a bright light, obtained in the daytime from the sun or 

 sky, and at night from any good lamp, effects like those just 

 alluded to as seen with candle flame, may be observed. After wit- 

 nessing the actions of one " grating," and one source of light, a 

 second grating may be imposed over the first and at right angles 

 to it. A brilliant chromatic star will then appear, with four 

 principal rays, and other intermediate rays, which become con- 

 spicuous in proportion to the brightness of the light. The 



