20 Prof. R. Threlfall on the Electrical 



riment. It was necessary, therefore, to make a map of the 

 spectrum to suit my spectroscope. The latter was an instrument 

 with a 5-in. Trouton and Sims circle, and was made by Elliott 

 Bros. ; one fine heavy glass prism was used throughout, and 

 the map refers to it ; it was always in the position of minimum 

 deviation for each line observed. The mapping of the spec- 

 trum was done for me by Mr. J. H. D. Brearley (a student 

 in the laboratory) , who also assisted at this time in all the 

 spectroscopic work and to whom my thanks are due. The 

 map was made by a selection of 33 lines drawn from all sorts 

 of sources, and verified in all sorts of ways, the tables used 

 being those in the B.A. Reports for 1884, 1885, 1886 ; Row- 

 land's map (got in 1889) ; and Rowland's table, Phil. Mag. 

 1889. Schellen was also found of some use. Each line 

 was set upon ten times ; and the prism was adjusted ten 

 times. 



Intermediate lines were now well picked up off Rowland's 

 map. Our limit of discrimination was about 2 in the fourth 

 significant figure of the wave-length, i. e. I could not decide 

 with certainty that a line I made out to be at, say, 5165 was 

 not at 5163. 



I append a list of the lines used in mapping the spectrum, 

 not because it is in any way to be regarded as a model selec- 

 tion, but because I, as a beginner at spectroscopy, should have 

 been very glad of such a list at the time, had I been able to 

 find one. I also note the means adopted by us for the iden- 

 tification of the lines, as it may interest other beginners in 

 spectroscopy. The lines are not given in the order in which 

 they were examined, but in order of wave-length. 



1.— B. Sun, 6867-796-6867-462. Rowland, Phil. Mag. 

 June, 1889. Recognized by being near both to a large 

 iron and to a potassium line. 



%.- — Li a , 6705*4. From lithium chloride in a Bunsen flame. 

 Wave-length from B.A. Report, 1884; given by Mascart, 

 Liveing and Dewar, and Thalen. 



3. — C. Sun (hydrogen). Good line recognized by com- 

 parison with a tube-spectrum of H 2 0. \= 6563*042. 

 Rowland, Phil. Mag. 1889. 



4. — Sun, 6400-2. A good line picked up when the map 

 was nearly finished from Rowland's map. 



5.— Sun, 6191'7. Same as number 4. 



6. — Lip, 6102. Authority, Mascart. 



7. — Iron line, 5930'4. Rowland's map verified by com- 

 parison with spark-spectrum of iron. B.A. Report. 

 Cornu gives 5929'3. 



