THE SPECTRUM 321 



maybe exhibited by Becquerel's phosphoroscope ; but Tyn- 

 dall's apparatus is simpler and more effective. A square box 

 A, shown in plan in fig. 174, is fitted either with an arc lamp 

 or magnesium burner, as already described, at c, and in one 

 side is a perpendicular slot B, all else being closed except a 

 sight-hole for regulation of the light. Outside the slit, on a 

 vertical axis, the wooden cylinder D revolves rapidly, driven 

 by bands E E from a multiplying wheel. The cylinder is 

 painted over with some strongly fluorescent substance. Pro- 

 fessor Tyndall used uranium glass powdered and laid on with 

 gum, but uranine in gelatine is much easier to use, and even 

 more effective. On rotating the cylinder, if there were no 

 persistence of effect, no radiation would be visible, the cylinder 

 being only directly illuminated through the slit ; but owing 

 to the duration of the vibrations set up by this momentary 

 illumination, the whole cylinder glows with the characteristic 

 fluorescent light. 



CHAPTER XX 



INTEBFERENCE OF LIGHT 



183. Propagation of Waves. This may be illustrated by 

 revolving the edge of a blackened glass disc, round the edge of 

 which is traced a sinuous wave-line, in front of a coarse 

 grating of perpendicular lines scratched on another piece of 

 blackened glass, after the method of Crova. But a slide 

 which I devised projects the phenomena in a manner both 

 simpler and better. It is shown in fig. 175. The grating A 

 of perpendicular lines scratched (pretty coarsely, and one- 

 sixteenth of an inch apart) on black-varnished glass, is 

 inserted flush with the surface of the slab of wood c which 

 holds the slide together, and which has an aperture cut behind 



y 



