22 Prof. R. Threlfall on the Electrical 



26. — Indium blue line. \ = 4509'6. Liveing and Dewar 

 and Thalen, B.A. Report, 1884. Took Liveing and 

 Dewar's number. Flame-spectrum. 



27. — Solar spectrum. \ = 4405. Rowland's map. Snel- 

 len's map gives it identical with an iron line. Verified 

 by spark. 



28. — Solar spectrum. X^4383'6. As in case of last line 

 (27). 



29. — Solar spectrum. Hydrogen violet. X = 4340*5. Row- 

 land's map and vacuum-tube. 



30. — Solar spectrum. \ = 4325'982. A fine line. Rowland's 

 map and Phil. Mag. 1889. Picked up from map. 



31. — Solar spectrum G. \ = 4308*023. Rowland's map and 

 Phil. Mag. The subject of a misprint in Rowland's list 

 in Phil. Mag. 1889. It is an iron line. 



32. — Solar spectrum. X = 4227. A calcium line given in 

 Schellen's map. Well seen on Rowland's map. Could 

 not verify from flame. 



It will be found that the lines mentioned in this list are 

 well spaced, and a curve drawn to them will enable most of 

 the other lines to be set upon by calculation. It is obvious 

 that all sorts of appliances must be to hand in order to verify 

 the lines in the first instance, as well as a good stock of the 

 rarer metals and their salts. 



Preparation of Pump- Gauge. — All gauges of closed U-type. 

 Gauge 1 constructed of tube 3 centim. in diameter, and 

 3 millim. thick in the walls, tested for evenness, &c. Each 

 limb about 20 centim. long, one limb closed at the top and 

 the other open to the pump. The bend constructed of thick 

 tubing having a bore of rather less than 1 millimetre in dia- 

 meter. This gauge was boiled out in a good Sprengel vacuum 

 some twenty times, but I never could get rid of the last 

 minute traces of gas. I have an idea that the last trace of 

 gas cannot be got rid of — it can only be reduced asymptoti- 

 cally. However, the gauge w T as excellent ; it was painted 

 black, except for a slit down each limb, and was read (with 

 the precautions mentioned by Regnault) by one of the Cam- 

 bridge Scientific Instrument Company's (Poynting's) cathe- 

 tometers from about three metres' distance. This gauge be- 

 haved admirably, and I used it during the spectroscopic 

 work ; in some of the preliminary tube-experiments at a low 

 pressure, however, a joint broke, and the momentum of the 

 mercury was sufficient to break the closed end of the gauge in 

 spite of the constricted bend. Under the circumstances, the 

 gauge being much better than was required, I made No. 2 



