Mr. J. S. Ames on some Gaseous Spectra, 49 



designed by Prof. Rowland for concave gratings, and de- 

 scribed in my paper in the Johns Hopkins University 

 Circular 73, 1889 *. Photographs were taken of the 

 gaseous and solar spectra on the same plate. Some of the 

 hydrogen plates were measured on a dividing-engine ; but in 

 most cases the wave-lengths were simply read off from the 

 coincidences between the gaseous lines and certain places in 

 the solar spectrum, the wave-lengths of which were taken 

 from Rowland's map. The error in any wave-length is, I 

 think, not more than 0*05 of a unit (i. e. 0005^), or about 

 one part in one hundred thousand. To guard against any 

 displacement of the gaseous spectra, the positions of certain 

 mercury-lines, which were present on all the plates, were 

 carefully noticed and measured. The complete arc-spectrum 

 of mercury has recently been photographed by Prof. Row- 

 land ; and so the lines are accurately located on the map. 

 No plate was used in the present work in which these lines 

 were not exactly in their proper places. Their wave-lengths 

 are: — 



2653-80 



2652-15 



3650-28 4077-98 



3654-96 4347-71 



3663-03 4358-50 



3663-41 4451-09 



4046-67 



Several forms of spectrum-tubes were tried, but that finally 

 adopted was the one shown in the cut (p. 50) . As is apparent, 

 it was used " end on." It consists essentially of two tubes 

 containing thin cylinders of aluminium and connected by a 

 finer tube. This tube was in one case circular, having a bore 

 of about 1 millim., and in another it was elliptical. This latter 

 form is preferable, because it keeps the light in the tube 

 better, and because it produces an elongated image on the 

 slit. The use of the aluminium cylinders was suggested by 

 Prof. Rowland. They were in connexion with an induction- 

 coil by means of platinum wires which entered each end-tube, 

 and which were bent so as to make good contact. The cylin- 

 ders make, as it were, a lining for the two end-tubes ; and 

 since, when the vacuum is high, the discharge takes place 

 from the whole surface of the electrode, the brilliancy of the 

 light in the capillary tube is much greater than in the ordi- 

 nary form of apparatus. A quartz plate was fastened to the 



* Also Phil. Mag. [5] xxvii. (1889). 

 Phil. Mag. S. 5. Vol. 30. No. 182. July 1890. E 



