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SUMMARY OF GUKRENT RESEAKGHES RELATING TO 



In arranging an experiment for this purpose, the chief essential was to 

 avoid, as far as possible, stray light from the wall of the vessel used as 

 an air-container, and to observe the beam transversely against the 

 blackest possible background. 



The source of light was a hand-regulated carbon arc, taking about 

 twelve amperes. A quartz condenser (used on account of its trans- 

 parency to blue and violet light) was adjusted to give a convergent 

 beam. This passed through a quartz water-cell to take our heat-rays, 

 then into the experimental vessel. The latter was in the form of a 

 cross, constituted of brass tubiug l|-in. diameter, as shown in the 

 figure (drawn to half -size). The interior was dead black. The light 



ARC 



PLATE 



entered by the quartz window A and passed on through the rectangular 

 diaphragm B to the far end, where it was stopped by the closed end of 

 the tube. The beam was observed through the glass window D. 

 The side-piece E constituted a black cave, and the mouth of it 

 afforded the necessary background against which a feeble transverse 

 luminosity of the beam would stand out well. In spite of all precau- 

 tions enough light was diffused by the walls of the tube to enclose this 

 black background in a luminous ring, which, however, was not bright 

 enough to cause inconvenience. The beam passed diametrically across 

 this luminous ring, which is seen in all the photographs. 



For photography the lens F, of 6 c.cm. focal length, was arranged 



