Mercury Vapour at the 2536 Absorption Line. 435 



could be very nicely controlled by varying the height of the 

 burner and the rate o£ flow of the water in the aspirator. 

 For higher temperatures the flame was placed just below 

 the aperture of the glass tube, so that the hot products of 

 combustion were drawn through the air-bath. 



It was found that the temperature could be held constant, 

 or rather within half a degree, for fifteeu minutes or more, 

 aud as exposures of two or three minutes were all that were 



necessary, this arrangement gave entire satisfaction. 



The 



Fig. 1. 



Thermom 



air-bath was wrapped up in cotton, and disks of black paper, 

 each perforated with a hole 1 cm. in diameter, were placed 

 over the ends. The beam of light passed through these 

 holes, traversing the quartz tube, which was mounted at the 

 centre of the air-bath. As I do not remember to have seen 

 any account of interferometer work in the ultra-violet, brief 

 mention of some of the difficulties may be helpful to others 

 taking up work along similar lines. The source of lio-ht was 

 the iron arc, which was rendered parallel or slightly con- 

 vergent for the ultra-violet, by means of a quartz lens. The 

 fringes were first found with sodium light and the centre of 

 the system found in the usual manner with a small o-as flame. 

 The mirrors of the instrument were adjusted so as to give a 

 system of four or five horizontal fringes across the field of 

 the instrument, which was of course somewhat constricted 

 by the perforated screens of black paper, used to prevent as 

 much as possible the escape of currents of hot air from the 

 air-bath. The iron arc was now placed in position behind 

 the sodium flame and its light focussed upon the back mirror- 



2H2 



