158 The Right Hon. Lord Rayleigh [May 7, 



spectrum are of quartz. I allow the spectrum to fall on a piece of 

 paper, and you see the usual succession of colours, red, yellow, green, 

 blue and violet, forming a comparatively narrow rainbow-like band. 

 Beyond the violet all appears dark, the eye being insensitive to the 

 ultra-violet rays. If now I substitute for the paper a screen of 

 barium platinocyanide * (of the kind used in X-ray work), we see an 

 immense extension of the spectrum beyond the violet. The screen 

 has the property of transforming the ultra-violet rays, which the eye 

 cannot detect, into green rays which are readily visible. Thus 

 beyond the violet region we see green, which is, of course, in no way 

 to be confused with the original green which was present in the 

 source, and appears in its normal position in the spectrum, on the 

 other side of the blue- violet. I interpose a thin sheet of ordinary 

 glass, and the greater part of this extension of the spectrum which 

 we get on the fluorescent screen disappears. What I want specially 

 to show you, however, is that a thin layer of ozone, much too thin to 

 have any perceptible colour, will have the same effect. There is a 

 glass tube, about 6 inches long and f-inch diameter, situated 

 between the quartz lantern condenser and the slit, when the beam- 

 is parallel, and the walls of the tube are projected as two thin 

 transverse lines on the slit, dividing the spectrum into thin horizontal 

 strips, one over the other. The light constituting the middle strip 

 has traversed the tube, but the light constituting the upper and 

 lower strip has traversed the open air above and below the tube. 

 A stream of oxygen passes through a Siemens' ozone generator and 

 enters the middle of the observation tube, streaming out at the two 

 ends. While the ozone generator is not excited, the middle strip of 

 the spectrum is similar to the comparison strips above and below. 

 If the induction coil is turned on so that ozone passes into the tube, 

 you see that in a few seconds the greater part of the ultra-violet 

 spectrum fades out from the middle strip, which contrasts sharply 

 with the upper and lower ones, When the coil is turned off, the 

 ozone is rapidly blown out by unozonised oxygen, and the original 

 state of things restored. 



It must be remembered that the ozone used in this experiment is 

 extremely dilute, probably only a fraction of 1 per cent, of the 

 oxygen in the tube. Yet it interposes an impassable obstacle to the 

 ultra-violet rays, at least to those of shorter wave-length than about 



2900 Angstroms. It cuts off the iron spectrum at about the same 

 point where the solar spectrum ends. Speaking roughly and generally, 

 it may be said that glass is somewhat more opaque than ozone, i.e. 

 that with diminishing wave-length the limit of transmission is 

 reached somewhat sooner. To make a statement of this kind quite 

 definite the thickness must of course be specified. 



Sir William Huggins devoted a great deal of attention to the 



* Kindly lent by Messrs. Watson. 



