802 Mr. H. Smith on the Spectroscopy of the Electric 



the discharge is made to pass through a very fine capillary 

 in the solution, the spectra of the capillary discharge having 

 quite distinct features from those of the brush spectra. 



These variations are discussed in the paper referred to, 

 but the observations were confined to the visible region 

 (X = 6700 A. to X = 4200 A.), and the brush in all cases was 

 alternating. The present research was undertaken to extend 

 the observations into the ultra-violet region and to investigate 

 the differences which may occur between the positive and 

 the negative brush. A large number of solutions was 

 examined, but observations were confined to the uncondensed 

 brash only. 



Apparatus. 



In fig. 1 is given a diagram of the apparatus used in these 

 experiments. The brush was produced in the cell, C, which 

 contained a litre of the solution under examination, and was 

 provided with a quartz window affixed with sealing-wax. 



Fi* 1. 



- - \u - 



6 



-^ i! 





WW 





Hi— 



f. — — ^j 



[%| 



li—jt 



Slit of 



Spectrograph 



An image of the brush was focussed by means of a simple 

 quartz condensing lens (3 in. focus) onto the slit of a small 

 quartz spectrograph. The potential difference between the 

 electrodes was measured by means of an electrostatic volt- 

 meter, V, and the current through the brush by means of a 

 galvanometer, G. The coil and outside case of the galva- 

 nometer were connected together and earthed in order to 

 avoid electrostatic effects. A Lodge valve, R, was used to 

 rectify the current, a positive or negative brush being pro- 

 duced simply by reversing the valve. The anode of the 

 valve was kept immersed in water, as it otherwise became 

 very hot during a series of exposures and was very liable to 

 crack. Instead of an induction-coil, a large transformer 

 immersed in oil was used. The spectrograph employed gave 

 a very small dispersion in the visible part of the spectrum ; 

 the total length of spectrum from X 6563 to X 2400 was a 

 little less than 4 - 5 cms. 



