334 Prof. Wood and Mr. Kimura on Scattering and 



exposure times (40 sees.) were given and the images 

 developed simultaneously. It is evident that the scattered 

 resonance radiation decreases (replaced by true absorption 

 probably) long before regular reflexion commences. 



Experiments on Selective Reflexion. 



In our first experiment on this subject we placed the 

 quartz bulb a little inside o£ the focus of the monochromator, 

 so that the incident radiations came to a focus after re- 

 flexion from the prismatic plate. A plate of uranium glass 

 was mounted in such a position that the two reflected images 

 were focussed on it. The image formed by reflexion from 

 the outer surface was noticeably brighter than the other, 

 owing to absorption by the fused quartz plate, which was 

 twice traversed by the rays reflected from its inner surface- 

 On heating the bulb to a red heat with a Bunsen flame, the 

 latter image brightened up until it appeared to be about three 

 times as bright as the image reflected from the outer surface. 

 In this way it is possible to demonstrate the selective reflexioni 

 of the vapour to a small audience at close range. A sheet 

 of heavy plate glass must be used as a protection against a 

 possible explosion of the bulb as the pressure may rise to 15 

 or 20 atmospheres. 



The reflecting power was next determined quantitatively 

 in the following way: — 



The total radiation from the water-cooled arc was reflected 

 from the inner surface of the prismatic plate into a small 

 quartz spectrograph, the slit of which was opened rather 

 wide, and shortened to a length of about 1 millimetre. 

 The spectrum lines thus photographed as small rectangular 

 patches, and twenty or thirty exposures could be made on a 

 single plate. The exposures were made by a slow swinging- 

 shutter of the pendulum type. We first made a series of 

 exposures of continuously increasing duration by operating 

 the shutter once, twice, three times, &c, with the bulb at 

 room temperature. This gave us a record of the reflecting 

 power of the inner surface of the quartz plate. The bulb 

 was then raised to a red heat, and another series of exposures 

 were made in the same way. 



The plate showed that the rectangle representing the 2536 

 line had the same intensity for an exposure of five seconds 

 for quartz reflexion (bulb cold), and one second for mercury 

 reflexion (bulb hot). 



Since the reflecting power of a surface of fused quartz is 

 roughly 5 per cent, in the ultra-violet, this experiment shows 



