from Long' Vacuum Tubes. 735 



with a collimating lens and a Cooke photographic objective 

 of one-metre focus. 



The wave- lengths of the lines of the Balmer series from 

 H^) to H 16 were determined from measurements made of a 

 plate taken of the 5th order spectrum, with an iron com- 

 parison spectrum. They are given in the following table 

 together with the values given by Dyson and Evershed for 

 the solar hydrogen lines : — 



Hyd. Wood. Dyson. Evershed (mean). 



10 3750-26 3750-27 



11 3734-47 3731-53 



12 3722-08 372206 3721-98 



13 ... 3712-06 —12-12 —1213 



14 3703-95 —04 —04-01 



15 3697-26 3697'29 —97-28 



16 3691-60 —91-70 —91-70 



17 3686-95 —87 —86-96 



18 3682-84 —8290 —82-94 



19 3679-39 —79-50 —7952 



20 3676-58 —76-54 — 76-51 



In my earlier paper I reported the 20th line as recorded. 

 Unfortunately the plate from which the old measurements 

 were made was destroyed by an accident, and I now feel 

 very doubtful about its appearance, for all of the more 

 recent plates show three lines of the new spectrum spaced 

 thus 18 \, and the position of the 20th line comes midway 

 between the close pair. If present it would scarcely be 

 resolved from the neighbouring close lines, and its detection, 

 if much fainter than these lines, would be difficult or impos- 

 sible. In the table I have given, however, the wave-length 

 of the point midway between the two lines as being that of 

 the 20th line. 



Oxygen lines are always present in the spectrum when the 

 Balmer series is most extensively developed, the most con- 

 spicuous ones being just below the 5th hydrogen line, and 

 just above the 16th (PI. XXII. spectra "a"). 



In regard to the decrement of intensity of the lines of the 

 Balmer series, as I showed in the earlier paper, the rate of 

 change varies as we pass down the series. The intensity 

 of the 6th line is about ^ that of H a , the 11th line . * 0Q of 

 H„ and the 20th line ^Voo" oi H - 



In the case of the nebulae the 11th line has an intensity of 

 about r J that of Il a . 



