100 Prof. A. F. Sundell's Researches 



lines instead of only one. Generally the spectrum-lines 

 were brought into coincidence with the central index-line 

 by turning the micrometer-screw ; only for three lines in 

 the extreme violet the outer index-line was used, because the 

 screw-thread ceased here. The wave-lengths were deter- 

 mined by means of a table which I had constructed from a 

 curve of wave-lengths belonging to the spectroscope, from 

 the difference between the position of a line and that of the 

 sodium-line. By way of control, the lithium-line 5706 and 

 the strontium-lines 4606 and 4305 were read in each series 

 of measurements, the corrections for the spectrum-lines mea- 

 sured being determined by simple interpolation. 



The larger spectroscope, belonging to the Physical Labora- 

 tory of the University, which I have employed for some 

 measurements, is also constructed according to Wrede's prin- 

 ciple. It has two prisms of heavy flint glass, and reflecting 

 crown-glass prisms. The index was constructed by H. Biese, 

 who had published the idea of a self-luminous index at about 

 the same time as myself*. The index consists of a fine slit in a 

 thin brass plate ; the side of the slit turned towards the prisms 

 is filled with a phosphorescent substance. The dispersion is 

 considerable ; ten divisions of the micrometer correspond to 

 a change of 0'0000013 in the extreme red and 0'0000002 

 millim. in the extreme violet ; the sodium-lines D 1 and D 2 

 are divided. The tables for the wave-lengths were calculated 

 from a curve drawn by H. Biese. 



Most of the observations were made in a dark room in the 

 Physical Laboratory of the Polytechnic Institute of Hel- 

 singfors. I take the opportunity of expressing my thanks 

 both to the Director of the Institute, H. Qvist, and to the 

 Professor of Physics, Dr. Slotte, both for the use of the room 

 so specially suited for these experiments, and also for the use of 

 various apparatus necessary for my experiments from the 

 collection of instruments belonging to the Laboratory. The 

 electrical machine was stationed in a neighbouring room, 

 where it was kept in action by means of a water-motor. For 

 the rarefaction of the gas in the spectral tube, I employed the 

 mercury-pump which I had constructed for the Polytechnic 

 Institutef. The movable tube had neither tap nor stopper, 

 and the globe with phosphoric anhydride was melted on. 

 The spectral tube employed was 155 centim. long, and at 

 one end had an internal diameter of 10*8 millim. and glass 

 2*1 millim. thick ; at the other end the tube was 12'9 millim. 



* Ofversiyt af Finska Vetenshaps-Societetens forhandlingar , xxiv. 

 p. 30. 



t Acta Societatis Scientiarum Foiniccc, vol. xv. p. 169. 



