52 



Dr. R. T. Beatty on 



Monochromatic light could thus be made to issue from the 

 emergence slit E in the position o£ minimum deviation. The 

 drum was engraved on its outer circumference with a scale 

 of wave-lengths from 3800 to 8000 A.U., the calibration 

 being carried out by observation of many well-known lines 

 in the spectrum. The essential parts of the instrument are 

 drawn to scale in fig. 2. E was shaped to the average cur- 

 vature of the spectrum-lines so as to obtain as much light as 

 possible, and the jaws of the slit could be removed to allow 

 of a photographic plate being mounted when desired. The 

 mirrors were silvered by cathode deposit, and could easily 

 be removed for re-silvering. 



The Geissler tube, the condensing lens, and the mono- 

 chromator were mounted on an optical bench. I was usually 

 opened to admit about 5 fju/x, and E to admit about 7 fi/j, of 

 the spectrum. 



The Photo-Electric Cell. 



A tube * of fused quartz (fig. 3) was provided with three 



Fig. 3. 



electrodes, whose points of entry were made air-tight by the 

 use of lead seals. The anode A is an aluminium rod, 2 mm. 

 diameter, fused on to the molybdenum wire which passes 

 through the quartz. C is the cathode and G a guard 

 electrode. The interior of the tube was thickly silvered, 



* This tube was made and the electrodes sealed in by the Silica Syn- 

 dicate, Hatton Garden, London. 



