444 



Mr. J. N. Lockyer. On the 



[Mar. 20, 



When observations of spot spectra were commenced in 1869, the 

 original idea was to observe the behaviour of every line widened or 

 ' brightened in the spectra of each spot. 



It was soon found, however, that in this climate it was exceptional 

 to do this completely on any one day. Still, when it can be done, it 

 is most important to secure such observations, and accordingly a com- 

 plete method of reduction of such observations was suggested, laid 

 before the Solar Physics Committee, and published by them in their 

 Report. 



Laboratory observations soon indicated the importance of having 

 a series of strictly comparable observations. It became obvious there- 

 fore that the observations would require to be considerably restricted. 



One reason why it was important to obtain such a series was that 

 they might be compared with the complete records of bright lines 

 seen in prominences given by Tacchini and others. 



This consideration led to the suggestion that it would be advisable to 

 take only the most widened lines, by which is meant the lines relatively 

 the most thickened in the spots : accordingly the six most widened 

 lines in each of the two regions, F to b, b to D, have been taken on 

 every available opportunity. lu this way a number of strictly com- 

 parable observations have been obtained. 



Besides these observations, attempts have been made to photograph 

 the spectra of sun-spots, and several photographs have been obtained. 

 In all H and K were seen reversed over the spots, just as Young saw 

 them at Sherman, while the blue calcium line was not reversed. The 

 dispersion employed, however, up to the present has not been sufficient. 



Previous researches had shown — 



1. That with increased density in the spot- vapours we might expect 

 an increase in the number of lines radiated, and therefore absorbed, 

 either in the case of one vapour or of a mixture of vapours. 



2. That an increased quantity of any one vapour in a mixture would 

 increase the number of lines visible in the spectra of that substance. 



3. That since absorption will vary with temperature, and as absorp- 

 tion is effected at different heights in the solar atmosphere, where the 

 temperatures are different, depending upon the height on the average, 

 the lines observed will vary according to the position of the absorbing 

 stratum. 



Information on all these points could be obtained by the method 

 proposed, if the lines belonging to each substance were separately 

 discussed afterwards. An individual discussion of each substance then 

 formed part of the plan of the work. 



Some General Conclusions. 

 1. In the photographs of spectra of sun-spots the H and K lines 

 were always reversed, while the blue calcium line was not. 



