332 Prof. Wood and Mr. Kimura on Scattering and 



We lirst investigated the diffuse resonance radiation as a 

 function of the density o£ the mercury vapour in the bulb. 

 The light of the quartz arc was passed through a quartz 

 monochromator arranged to give a convergent cone of 2536 

 monochromatic light. It was simply a roughly constructed 

 quartz spectroscope with a very wide slit and no telescope- 

 tube, shown in diagram in fig. 2. The image of the slit 



Fig. 2. 



formed by the 2536 rays was located in space by means of a 

 strip of uranium glass, and the bulb mounted in such a 

 position that the image fell upon the centre of the prismatic 

 plate. The dispersion was sufficient to remove the other 

 images of the slit from the bulb, which obviated the use of a 

 second slit and lens for obtaining the monochromatic illumi- 

 nating beam. The quartz bulb was mounted over a chimney 

 of thin sheet iron, with a Bunsen burner at its base, and the 

 temperature determined by a nitrogen-filled mercury thermo- 

 meter, the bulb being in contact with the upper surface of the 

 quartz bulb. A camera of very simple construction, furnished 

 with a quartz lens, was focussed upon the bulb, the process 

 consisting in first focussing it with uranium glass upon the 

 image in space of the 2536 line formed by the monochro- 

 mator, and then measuring the distance between the lens 

 and the image in space. This gives us the proper distance of 

 the bulb from the lens to secure a sharp focus. 



The arrangement of the apparatus in this experiment is 

 shown in fig. 2, the rays reflected from the two surfaces 

 of the prismatic plate falling to one side of the lens. In this 

 w r ay we obtain only an image of the scattered resonance 

 radiation from the bulb. 



The photographs obtained at different temperatures are 



